All you have to do is set RadioButton.Checked = true in the DropDownList.SelectedIndexChanged event handler for your corresponding controls. To find out which index is selected, just check the DropDownList.SelectedIndex property.

In DropDown.SelectedIndexChanged you have to place the code of radiobutton1.Checked=True. But before that you have to set autopostback= True for DropDownlist.

Hope this can help You.