Well if you do no calculations at all, the smallest number you
could narrow the answer down to is 6 because the denominator is
a 6th degree equation, which could possibly have as many zeros
as the degree, that is, 6.  

So I would guess 6, answer c).

However, with more calculations you can narrow it down to only 2,
and that would be choice b).  

However, even though you can narrow down the answer to 2 by
additional calculation, I'm pretty sure your teacher would want
you to answer c).

Edwin