You can achieve that using the DataBound event of the DropDownList. Here is an example.. In your aspx page you need to add the OnDataBound property like this:
<asp:DropDownList ID="ddlOptionDependant" runat="server" AutoPostBack="True"
DataSourceID="sdsOptionDependant" DataTextField="product_option_name"
DataValueField="product_option_id" AppendDataBoundItems="True"
OnDataBound="ddlOptionDependant_DataBound">
<asp:ListItem Value="0" Text="None"></asp:ListItem>
</asp:DropDownList>
</asp:Content>
And in your code behind add this:
protected void ddlOptionDependant_DataBound(object sender, EventArgs e)
{
//Get the value of the ID you want to match here:
int someId = 1;
foreach(ListItem item in ddlOptionDependant.Items)
item.Selected = item.Value == someId.ToString();
}
Hope this helps you