In my previous comment that I asked to ignore about events, still the line -- drpFilter.Items.Clear(); -- is valid and you should try it. In your code it will be like:
public DataSet PopulateDropDown()
{
drpFilter.Items.Clear();
string sSearchType = string.Empty;
if (rdba.Checked == true)
{
//Bind Drop down
dsFit.dataSource
}