change
Response.Redirect("~/View Products.aspx?MenuId=" +Menu1.SelectedItem.Value);
to
Response.Redirect("~/View Products.aspx?MenuId=" +e.Item.Text);
and also use parameterized sql statement
int id;
if (!int.TryParse(Request.QueryString["MenuId"], out id))
{
string sql = "select * from rsa_ProductItemTable where MenuId=@id";
SqlDataAdapter da = new SqlDataAdapter(sql, con);
da.SelectCommand.Parameters.AddWithValue("@id", id);
DataSet ds = new DataSet();
da.Fill(ds);
DataList1.DataSource = ds;
DataList1.DataBind();
con.Close();
}