Voodoo
Be careful. The exact text of the first sentence is:

Some observers have attributed the dramatic growth in temporary employment that occurred in the US during the 1980s to the increased participation in the workforce by certain groups, such as first-time or reentering workers, who supposedly preferred such arrangements.

The commas are there primarily to offset the example phrase "such as first-time or reentering workers". Since the commas are there for another reason, it's hard to tell simply from the punctuation --- is the modifier "who supposedly preferred such arrangements" vital or non-vital, that is, restrictive or descriptive? Often, punctuation alone is often not enough to tell, because other things, like example phrases, have their own punctuation requirements. You always have to consider both the logic of the sentence and the logic of the passage overall.

Well, drop the example part and drop the modifier -----
"...to the increased participation in the workforce by certain groups."
Do we have any idea who these "certain groups" are? Does this noun add to the logic of the sentence? NO! If the noun's identity or logical role in the sentence is unclear without the modifier, that's a sure sign the modifier is a vital modifier.

Now, leave the example out, but add the modifier as a vital, restrictive modifier.
"...to the increased participation in the workforce by certain groups who supposedly preferred such arrangements."
OK, now that group is identified and their logical role in the sentence is quite clear.

Let's back up to the larger view of the passage overall. The opening phrase "Some observers have attributed..." is a HUGE hint --- that's the author's way of saying: "I'm about to state a view with which I entirely disagree, for the purposes of shooting it down." The author of the passage clearly feels that the inordinate number of temporary work positions is unfair and constitutes employers' taking advantage of workers. People are stuck in temporary jobs they don't want -- they want permanent jobs, but can't get them. This opening sentence, in which the author quotes the idea with which she disagrees, is going to be a strong contrast to this point. Essentially, the observers in the first sentence are saying: "so many people have temporary jobs because all those people want temporary jobs." Thus, in the large logic of the passage overall, the idea of "wanting temporary jobs", or as the passage states is, "who supposedly preferred such arrangements", is not a decorative phrase that could be easily dropped --- rather, it is part and parcel of the larger argument the passage is making. That modifier contains another hint word, "supposedly", which convey the fact that the author strongly disagrees with this entire claim --- another indication that the modifier is part of the logical apparatus of the argument overall, not merely decorative.

Thus, the phrase in the passage is restrictive, is vital, and the GMAT gods are 100% justified in interpreting it that way in question #54.

Does all this make sense?

Mike
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Mike McGarry
Magoosh Test Prep