正确答案：CF
解析：
The case for extraterrestrial life depends on more than just the number of planets ("...more than a...planets). Thus, the discovery of new planets only indirectly supports (which is the meaning of undergird) the case for intelligent life. Whether the planets have a viable atmosphere more directly relates to the question of intelligent life forms.
(A) and (B) somewhat make sense. But they create slightly different sentences meaning-wise.