Follow the code below:
HTML:
<asp:RadioButtonList ClientIDMode="Static" ID="rbAtivoInativo" runat="server" RepeatDirection="Horizontal" AutoPostBack="true"> <%--OnSelectedIndexChanged="rbAtivoInativo_SelectedIndexChanged"--%>
<asp:ListItem Value="ATIVO" Selected="True">Sim</asp:ListItem>
<asp:ListItem Value="INATIVO">Não</asp:ListItem>
</asp:RadioButtonList>
I wanted to see a div that is hidden when the RadioButton with the INACTIVE value was selected.