Validate dropdown using requiredfieldvalidator in asp.net

<div>

            <asp:DropDownList ID="DropDownList1" runat="server">

                <asp:ListItem Value="0" Selected="True">Select Letter</asp:ListItem>

                <asp:ListItem Value="A">A</asp:ListItem>

                <asp:ListItem Value="C">B</asp:ListItem>

                <asp:ListItem Value="C">C</asp:ListItem>

            </asp:DropDownList>

            <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="DropDownList1" InitialValue="0"
SetFocusOnError="true" ErrorMessage="Select the Letter"></asp:RequiredFieldValidator>

            <br />

            <asp:Button ID="btnSubmit" runat="server" Text="Submit" />

</div>

Note:- 

Server Error in '/' Application.

---

WebForms UnobtrusiveValidationMode requires a ScriptResourceMapping for 'jquery'. Please add a ScriptResourceMapping named jquery(case-sensitive).

If you are getting above error…

Copy past following code in web.config under <configuration> section.

<configuration>

  <appSettings>

    <add key="ValidationSettings:UnobtrusiveValidationMode" value="None" />

  </appSettings>

</configuration>