try this on selected index changed event of dropdown list
protected void d_selected_indexchanged(object sender, EventArgs e)
{
string str = "Select *from " + DropDownList1.SelectedValue.ToString().Trim() + "";
SqlCommand cmd = new SqlCommand(str,Db.GetConnection());
SqlDataAdapter ad = new SqlDataAdapter(cmd);
DataTable dt = new DataTable();
ad.Fill(dt);
GridView1.DataSource = dt;
GridView1.DataBind();
}