The molar percent of a in stream 3 is mol% a in stream 1.

From the problem description you've given, I can't give an exact answer without a context. However, I found a similar problem with an illustration. Supposing this is the problem, we can set a mole balance:

n₁ = n₂ + n₃

Since n₃ is just part of n₁, it will always be less than n₁ because n₁ constitutes 100% mol A. Thus, the answer is letter C.