Поправка, вариант 2

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApplicationDemo
{
    public partial class Popravka2 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        // 1
        protected void Button1_Click(object sender, EventArgs e)
        {
            if (CheckBox1.Checked)
            {
                TextBox2.Text = TextBox1.Text;
            }
            else
            {
                TextBox1.Text = TextBox2.Text;
            }
        }

        // 2
        protected void Button2_Click(object sender, EventArgs e)
        {
            ListBox1.Items.Clear();
            ListBox1.Items.Add(new ListItem("+"));
            ListBox1.Items.Add(new ListItem("-"));
            ListBox1.Items.Add(new ListItem("*"));
            ListBox1.Items.Add(new ListItem("/"));
            ImageButton1.ImageUrl = "Image.jpg";
        }

        // 3
        protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            var value1 = double.MaxValue;
            var value2 = double.MaxValue;
            if (double.TryParse(TextBox1.Text, out value1) && double.TryParse(TextBox2.Text, out value2) && ListBox1.SelectedItem != null)
            {
                Label1.Text = $"{value1 + value2}";
                if (ListBox1.SelectedItem.Text == "-")
                {
                    Label1.Text = $"{value1 - value2}";
                }
                else if (ListBox1.SelectedItem.Text == "*")
                {
                    Label1.Text = $"{value1 * value2}";
                }
                else if (ListBox1.SelectedItem.Text == "/")
                {
                    Label1.Text = $"{value1 / value2}";
                }
            }
        }

        // 4
        protected void Button3_Click(object sender, EventArgs e)
        {
            var intNumber = 0;
            if (int.TryParse(TextBox1.Text, out intNumber))
            {
                BulletedList1.Items.Clear();
                for (int i = 1; i <= intNumber; i++)
                {
                    BulletedList1.Items.Add(new ListItem($"{i}"));
                }
            }
        }
    }
}