Hi Priyank,
Please have a look into the sample code snippet to achieve your scenario.
ASPX:
<telerik:RadListBox ID="RadListBox1" runat="server" SelectionMode="Multiple" OnClientSelectedIndexChanging="OnClientSelectedIndexChanging1">
<Items>
<telerik:RadListBoxItem Text="Item1" />
<telerik:RadListBoxItem Text="Item2" />
<telerik:RadListBoxItem Text="Item3" />
</Items>
</telerik:RadListBox>
JavaScript:
<script type="text/javascript">
function OnClientSelectedIndexChanging1(sender, args) {
if (args.get_item().get_selected())
args.get_item().set_selected(true);
}
</script>
Hope this will helps you.
Thanks,
Shinu.