It's pretty simple.Use
SelectedIndexChanged
event of dropdownlist
Try this:
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e) {
if (DropDownList1.SelectedItem.Text == "Monthly")
{
ModalPopupExtender2.Enabled = true;
ModalPopupExtender2.Show();
}
else
{
ModalPopupExtender2.Enabled = false;
ModalPopupExtender2.Hide();
}
}