NASA’s methodical approach to the exploration of Mars has not yet answ

The parallelism looks pretty OK to me here. "NASA’s methodical approach to the exploration of Mars has not yet answered the questions of whether biological life forms ever emerged on Mars and whether... those traces can still exist....

The trouble appears to be that pesky little modifier in the middle of the underlined portion: "...presuming such life forms having left fossil traces to begin with..." It would be fine to say "presuming such life forms left fossil traces to begin with" -- but I can't make any sense of the phrase "having left" in this case. You could say something like "having left a mess in his pigpen, Wilbur proudly trotted off to make a mess somewhere else" -- the "having + verb" needs to be the first of two actions, as it is in my silly example. But I can't come up with any interpretation of "having left" in (A) that would make any sense at all, especially when one of the answer choices is clearly much better.

Plus, there's no good reason to use the word "can" in this situation. Is NASA trying to figure out whether the fossil traces can still exist -- or does NASA just want to know whether the fossil traces actually still exist? I'd argue for the latter.

So we can get rid of (A).

Well, this is a hot mess: "NASA’s methodical approach to the exploration of Mars has not yet answered the questions of whether biological life forms ever emerged on Mars and whether... can those traces still exist.... Um... "whether can those traces"? Seriously? That's ridiculous -- and the word "can" doesn't make any sense in this context anyway, as discussed in answer choice (A).

The modifier in the middle of the underlined portion is a much smaller problem, but it's still a problem: "if it is presumed" isn't nearly as clean as just "presuming." But that's a minor issue.

(B) is out.

Much better! The heart of the parallelism is completely fine here: "NASA’s methodical approach to the exploration of Mars has not yet answered the questions of whether biological life forms ever emerged on Mars and whether... those traces still exist.... No problem. And it's great that the word "can" has been removed, too.

The modifier in the middle of the underlined portion is also much better than the versions in (A) and (B): "presuming such life forms left fossil traces to begin with" makes sense here. Or at least more sense than the alternatives. Keep (C).

We have some issues here, even if we ignore that modifier in the middle of the underlined portion: "NASA’s methodical approach to the exploration of Mars has not yet answered the questions of whether biological life forms ever emerged on Mars and...if they can still exist.... The first problem is that there's no good reason to use "if" after the "and." I suppose that you could argue that it's not parallel to "whether", but more importantly, I'm pretty sure that "if" can only be used in a conditional statement (otherwise known as an "if-then statement": "if I eat fourteen dosas and seven kati rolls, then I will be happy...") on the GMAT, and (D) isn't a conditional statement. In these situations, the GMAT always seems to prefer "whether" over "if."

Plus, you could argue that the pronoun "they" is ambiguous, because it could plausibly refer back to "life forms" or "fossil traces." Pronoun ambiguity isn't an absolute rule on the GMAT, but it's also not ideal if you can avoid it.

And then we still have the issue with the word "can", as described in (A). So we can get rid of (D), too.

(E) has two of the same errors as (D): "they" is arguably ambiguous and "if" is not as good an option as "whether." Plus, there's no reason to say "presuming such life forms to have left fossil traces to begin with." Just "left" would have been fine.

So (E) can be ditched, and (C) is the best we can do.