C appears to be the best answer.
The trick to analyzing this CR is to bare it down to its bones:
(1) On earth, methane goes hand in hand with methanogens (bacteria).
(2) We found methane in extraterrestrial (ET) objects, i.e. comets.
(3) Thus, there must be methanogens in the ET objects.
Here's how the choices (A) and (C) fare:
(A) says: Argument is supported because we found ethane (associated with ethanogens on earth) in ET objects.
Wrong. If no bacteria exists in the ET objects, we may still get the same results. Why? Because, what if the bacterial action is associated with compounds only on earth? Keep doing the experiments, and you'll keep getting the same results. Try alcohol, colestrol, tomato ketchup....
OK, so how about you instead first analyze what it is that associates bacteria with these compounds on earth? Perhaps if you can explain to me that the association is independent of the earth's atmosphere and can exist anywhere in the universe, I'm more willing to listen. Which is what (C) does.
Here's the revised structure:
(i) On earth, methane <=> methanogens.
(ii) In ET objects, we found methane.
(iii) Methane is proven to associate with methanogens anywhere in the universe.
(iv) Thus, ET objects contain methanogens.
No. (iii) is what (C) attempts to identify. So, (C) is the best answer.