Hi
First if you have done engineering from core branches then choose them as your optional subject as you have studied it in your graduation and it will certainly help you but if you are looking for any other subject choose it on basis of your interest this is most important because you don't know how long it will take you to clear this exam so choose a subject that you can study for long hours and duration. Generally other subjects chosen by engineer's are sociology, public administration, geography and anthropology.
Hope it helps .....