Inheritance - Inconsistent Accessibility field type

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace AssignmentQ2
{
    public partial class Form1 : Form
    {
        List<Advisor> advisorList;                          //call  List<Advisor> at class level
        List<Students> students = new List<Students>();      //call List<Students> at class level

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            advisorList = Advisor.Load();
            if (advisorList == null)
            {
                advisorList = new List<Advisor>();
            }
        }

        private void addAdvisorButton_Click(object sender, EventArgs e)
        {
            Advisor a;

            frmAddAdvisor formAdd = new frmAddAdvisor();

            formAdd.ShowDialog();
            a = formAdd.newAdvisor;

            if (a != null)
            {
                advisorList.Add(a);
            }
        }

        private void viewAdvisorbutton_Click(object sender, EventArgs e)
        {
            frmViewAdvisor formAdvisor = new frmViewAdvisor();

            formAdvisor.listAdvisors = advisorList;
            formAdvisor.ShowDialog();



        }

        private void saveButton_Click(object sender, EventArgs e)
        {
            Advisor.Save(advisorList);
        }

        private void addStudentButton_Click(object sender, EventArgs e)
        {
            frmAddStudents s = new frmAddStudents();

            s.listAdvisor = advisorList;
            s.ShowDialog();
            students.Add(s.student);
        }

        private void viewStudentButton_Click(object sender, EventArgs e)
        {
            frmViewStudents s = new frmViewStudents();

            s.listAdvisors = advisorList;
            s.student = students;

            s.ShowDialog();
        }
    }
}







//this is  another form 

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace AssignmentQ2
{
    public partial class frmAddAdvisor : Form
    {
        public Advisor newAdvisor;   //call Advisor at class level class and declare newAdvisor as variable

        public frmAddAdvisor()
        {
            InitializeComponent();
        }

        private void cancelButton_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void addButton_Click(object sender, EventArgs e)
        {
            //calling Advisor class and using overload constructor to execute
            newAdvisor = new Advisor(advisorFirstNameTextBox.Text, advisorLastNameTextBox.Text, departmentTextBox.Text);

            newAdvisor = new Advisor();

            newAdvisor.FirstName = advisorFirstNameTextBox.Text;

            this.Close();
        }
        
        
    }
}



//Advisor Class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace AssignmentQ2
{
    class Advisor
    {
        //field
        private string _firstName;
        private string _lastName;
        private string _department;
        private Students _students;   //assigne Students class as variable

        //default constructor
        public Advisor()
        {
            _firstName = "";
            _lastName = "";
            _department = "";
        }

        //overload constructor
        public Advisor(string fn, string ln, string dep)
        {
            _firstName = fn;
            _lastName = ln;
            _department = dep;
        }

        //another constructor???
        public Advisor(string fn)
        {
            _firstName = fn;
        }

        //property for Students
        public Students _Students
        {
            get { return _students; }
            set { _students = value; }
        }

        //property FirstName
        public string FirstName
        {
            get { return _firstName; }
            set { _firstName = value; }
        }

        //property LastName
        public string LastName
        {
            get { return _lastName; }
            set { _lastName = value; }
        }

        //property department
        public string Department
        {
            get { return _department; }
            set { _department = value; }
        }


        //Save method for advisor list
        public static void Save (List<Advisor> listAdvisors)
        {
            AdvisorList.save(listAdvisors);   //use AdvisorList class and save method to execute this part
        }

        //Load list
        public static List<Advisor> Load()
        {
            return AdvisorList.Load(); //use the AdvisorList class and load method to execute
        }

        //get information
        public string GetInfo()
        {
            return "First Name: " + _firstName + ", Last Name: " + _lastName + ", Department: " + _department;
        }

        
    }
}

It says that my new Advisor (from form AddAdvisor) is inconsisten accessbility field type.

I have other form that also holds the same error on Class variables.

Can someone have an idea how to fix this?