Try this code:
<asp:radiobuttonlist id="RadioButtonList1" runat="server" autopostback="true" xmlns:asp="#unknown">
onselectedindexchanged="RadioButtonList1_SelectedIndexChanged">
<asp:listitem text="Enable Text"></asp:listitem>
<asp:listitem text="Enable DDL"></asp:listitem>
</asp:radiobuttonlist>
<asp:textbox id="TextBox1" runat="server" xmlns:asp="#unknown"></asp:textbox>
<asp:dropdownlist id="DropDownList1" runat="server" xmlns:asp="#unknown"></asp:dropdownlist>
protected void RadioButtonList1_SelectedIndexChanged(object sender, EventArgs e)
{
if (RadioButtonList1.SelectedItem.Text == "Enable Text")
{
TextBox1.Enabled = true;
DropDownList1.Enabled = false;
}
else if (RadioButtonList1.SelectedItem.Text == "Enable DDL")
{
TextBox1.Enabled = false;
DropDownList1.Enabled = true;
}
else
{
TextBox1.Enabled = false;
DropDownList1.Enabled = false;
}
}