i want to prepare for IAS in physics(1st sub). if i want to take any Art subject as 2nd optional sub, then plz suggess me, which should be?

Perhaps Geography would suit you better as it has many technical aspects which can be learnt comparatively with more ease by a person who has graduated in Physics.

you can choose a subject for your choice which you feel appropriate according to your taste. It may be history or psychology or any other subject