Hi, there. I'm happy to help with this.
The first split in the answers is the way the verb is completed. The auxiliary verb "have" can be followed by "seen", "been able", or "been seeing" (although that last is ridiculously wordy). The "see" of (B) is completely wrong.
(C) is absurdly wordy with "have been seeing" --- there's absolutely no reason for the past progressive in this context. (C) is out.
(D) is also unnecessarily wordy. Think about it.
Folks who have been to the Grand Canyon have seen the colored layers of sediment. Plain and simple. To say ----
Folks who have been to the Grand Canyon have been able to see the colored layers of sediment. ----- that adds extra words without adding any extra meaning. In GMAT terms, it's unacceptable to lengthen a sentence for no purpose. The only reason to introduce "have been able to" would be if the subject of the sentence itself is about establishing whether folks have the ability to see it at all. If it were in doubt whether anything could be seen, then establishing that folks were
able to see something is meaningful. Here, in this context, there's no doubt about one's ability to see the Grand Canyon. It's perhaps the single most photographed natural feature in the USA.
Everyone has seen it. There's no reason to raise the question about whether folks are able to see it. (D) is out.
(A) & (E) are close, but the odd passive construction in (E) --- "marking by different colors" --- is weak and indirect. (A) is the best answer, and I believe this is the OA.
Does this make sense? Let me know if you have any further questions.
Mike
Thanks for this explanation. Just one quick thing, I think there's a typo in option B where seen is misspelled as see. I would love to know any other decision point to eliminate option B in the Qs.