Ganesh, can you help me clarify your question a little bit? Is your question referencing the Service Desk product or ALM?
It's with Service desk... Incident Management
If I understand your question correctly it sounds like you need to modify your Incident process to include one or more conditions that test on the Category value. If they match the one(s) that you want an email for then the YES branch goes to an automatic action which is a Reminder (email). If they dont match then the NO branch of the condition links back to the next point in the process after the point where you inserted the condition (ie continue).
If you want emails for more than one category then you'll need to chain individual conditions (on the NO branch of the previous condition) as they can generally only test on a single value. If you are on 7.3 or later you can use a single condition which is of type "calculation" and put all the tests in the calculation.
Maybe that helps?