What option is the answer to the question?

This sounds like a question that could be solved by reading your book.  Depending on how deep your class is, if you have a reaction that goes to completion, you will have no extra atoms.  That sounds about right for this question.  If you are studying reactions that don't always go to completion, they may seek out other atoms to combine with unless they are stable.

The answer is a) carbon-12

Cause its being chemically applied?