AlexGmat2017
What about if we omit the "comma" in the 1st choice? If so, it means that clause "Unsure..." modifies the word "student". But if we have "comma" the clause "Unsure..." must modify the verb "stare". Where is the right answer?
AlexGmat2017 (no, we don't want to remove the comma, though I can see why you might think so

) and
puneetb - Just curious: what did you dislike about option D?
•
Unsure is an adjective that modifies
studentsIntro phrases that begin with (or are anchored by) adjectives modify the subject of the immediately following clause.
Here are a couple of official examples constructed with that pattern:
OG VR 2019 #219,
HEREOG 13 #88
HERE(Dozens of examples exist. Those two came to mind quickly.)
•
Unsure to whom is not idiomatic and if used at all, the phrase is coupled with a linking verb (is, are, become, seem, appear)
Unsure to whom is almost never used in high-level prose. On rare occasions it is accompanied by a linking verb.
In respected publications, the construction appears this way:
[T]he consultants found that 38 percent of officers who went to Lower Manhattan said that they were unsure to whom they should report.*
(I found a mere two examples of the phrase in the 10 respected publications that I checked.)
Option (D) states "they were unsure to whom."
That usage is consistent with what I described above.
AS means "because."
In order to avoid this discussion about "unsure to whom," I previously advised a focus on causation.
(D) has issues, too. AS could mean "during the time that."
In general, GMAC uses "because" at the beginning of a sentence similar to this one, not "as."
This question is not official. Do not worry about it. Finding SC questions is hard. Some will be flawed. This one is flawed.
Allegedly the question tests whether a test taker can recognize when to use
who and when to use
whom.
I will change the answer to (A) because now I am almost certain that you will never see "unsure to whom," and if the point is simply to test whether a person knows how to use who or whom, (A) does the job.
• I am 99.9 percent certain that you will never see "unsure to whom."
• In my research, I found
-- zero official questions that use "unsure to whom."
--
one official question that uses "unsure about."
That GMAT Prep question is
HERE I hope that helps.
*Unsure is an adjective that takes a preposition when used as it is in this question. That preposition is not "to."
Some adjectives automatically take certain prepositions.
Unsure takes about and of.
Unsure also takes WH- words such as whether.
Quirk et al. A Comprehensive Grammar of the English Language. [2010 (1985)]. §§ 16.68-73
** Rashbaum, William K. (2002, July 27). Report on 9/11 Finds Flaws In Response of Police Dept. New York Times 27 July 2002. Retrieved from https://www.nytimes.com (here, 23 June 2019)