На aspx-странице:
<asp:RadioButtonList ID="RadioButtonList1" runat="server">
<asp:ListItem>A</asp:ListItem>
<asp:ListItem Selected="True">B</asp:ListItem>
<asp:ListItem>C</asp:ListItem>
<asp:ListItem>D</asp:ListItem>
</asp:RadioButtonList>
В коде позади:
Dim item As New ListItem("A", "A")
Me.RadioButtonList1.Items.Add(item)
item = New ListItem("B", "B")
item.Selected = True
Me.RadioButtonList1.Items.Add(item)
item = New ListItem("C", "C")
Me.RadioButtonList1.Items.Add(item)
item = New ListItem("D", "D")
Me.RadioButtonList1.Items.Add(item)
или
Me.RadioButtonList1.SelectedValue = "B"