Priyanka, if the workflow tasks are from chg category, you would need to send notification using the behavior(the default action is "Action" so when you search for action, change the type to "multiple notification" otherwise you will see those actions only which does not give you chance to associate a multiple notification to a behavior); if the workflow tasks are
manually inserted/deleted/attached, you could use one of
delete workflow task
insert workflow task
workflow tasks attached
activity notifications to send email notifications. Thanks _Chi