Friend as you mentioned that you are commerce graduate, then you would be well aware from Commerce and Accountancy subject, so you should choose this subject as optional subject.
This subject will be helpful to crack Civil service exam.
On the place of one other subject you choose economics because it is related from commerce up to some extent.
So don�t think to take risk for Civil Service exam because its exam pattern is very tough and you should choose only those subjects where you are interested and well familiar.
There are several subjects to choose in UPSC mains exam. I am giving you the list of optional subject here and you can choose the relevant subject as per your educational profile.
Animal husbandry & vetinary science
Political science & international relations
Exam pattern for civil exam
You have need of great dedication and hard work to crack CSE.
For more information visit http://upsc.gov.in