In workflow need mandatory drop down field with multiple value in Reject case

 In my custom workflow if some user is selecting the task  assigned in workflow and  trying to reject the workflow, in that scenario we need a drop down appears in the (Complete Work Item) dialog box above the comment section. where the user can choose one of the relevant  reason from the drop down to reject the workflow.
And selecting the reason should be mandatory, without selecting any value from the drop down the user cannot reject the workflow.