Critical Reasoning

remember that you're looking for REQUIRED ASSUMPTIONS.

here's a very useful criterion to use in these problems:
try REVERSING putative assumptions and see the effect on the argument.
if you REVERSE A REQUIRED ASSUMPTION, the ARGUMENT SHOULD BECOME INVALID.

let's try this with choice (d):
reverse the assumption: let's say the average salary for non-degree jobs will increase over the next 4 years.
this would actually STRENGTHEN the argument (!), because it would provide another reason that the average salary would increase.
this is the complete opposite of the effect you're looking for; reversing the assumption should destroy the argument.
bad.
very bad.
wrong answer.

So I chose D, which could be another reason.

you can't introduce some random hypothetical consideration, which may or may not be true (and has zero evidence in the passage), and then try to criticize it as if it were a necessary component of the argument!
you should know better than this; making any unwarranted assumptions on CR is disastrous - let alone using those unwarranted assumptions as the basis for further reasoning.
this is one form of a common logical fallacy called ignoratio elenchi, if you're interested in such things.

as soon as you start saying "maybe it is possible that..." or "this could be..." in a Find the Assumption question, you should smack yourself in the head and yell "STOP!" at the top of your lungs.

--

funny analogy:
the test instructions don't tell you that you should continue breathing normally during the test, either. "but maybe it's possible that some students could hold their breath during quant questions!" maybe indeed.

kanha81 wrote:what else I noticed that I think is wrongly identified in [C] is the internal comparison between higher average salaries of workforce with degree jobs and average salaries of workforce with degree jobs.

these are indeed different, but they are related.

if you erase the differential between the different salaries (this relates to the whole idea of "internal comparison / higher average salaries"), then the overall average salary can't change anymore, because degreed and non-degree jobs would have reached equilibrium (i.e., similar average salaries).
therefore, you need the assumption that the differential between salaries will persist, even with the increased entry of degreed candidates into the work force.

again, realize that (d) is the EXACT OPPOSITE of the sort of assumption you'd want to make. if (d) DIDN'T happen, it would HELP you.

an ASSUMPTION has to be ABSOLUTELY VITAL to the passage. in other words, if the Assumption is FALSE, then the entire passage's validity is DESTROYED.

proceed accordingly.