Refering the below article able to populate dropdownlist.

Populate (Bind) ASP.Net DropDownList using jQuery AJAX and JSON in C# and VB.Net

This is been resolved, of increase json length at webconfig file.

<System.web.extension>
   <scripting>
       <webservices>
            <jsonSerialization maxJsonLength = "21453654" />
       </webservices>
   </scripting>
</system.web.extension>