There are two way to achive this
First in Aspx page
<asp:DropDownList ID="Sex" runat="server">
<asp:ListItem Selected="True" Value="Male" Enabled="true" >Male</asp:ListItem>
<asp:ListItem Value="Female" Enabled="true">Female</asp:ListItem>
</asp:DropDownList>
Second in CS page
foreach (ListItem i in Sex.Items)
{
i.Enabled = true;
}