Here are my thoughts while doing this question:
Argument:
Countries in labor shortages drive away workers at 65
-->
Labor shortages could be averted if this practice was eliminated
Thought process
The big assumption here is that workers wouldn't leave on their own. Just because workers are asked to leave doesn't mean that they would want to stay anyway. Some people retire at 65, not because they are asked to, but because it is generally accepted as the "time" to retire. In addition, the argument is assuming that labor shortages as a whole would be averted if these particular workers stayed. Perhaps these workers were not enough to compensate for the labor shortage. This may be something to think about when looking at the answer choices.
(A) This may be true, sure, but this doesn't really have any bearing on the argument. We don't care about how valuable their skills are. We are talking about labor here! It seems that, in order to avert labor shortages, the countries need LABOR and not just EXPERIENCE
(B) Out of scope. Why does it matter about what they are prepared or unprepared to do?
(C) Correct. This is what I anticipated because it is saying that a "large number of workers" (keep in mind that this not a sufficient assumption and this number may not be sufficient to stop a labor shortage) would still continue to work. If this is so, then this bridges the gap between the premise and the conclusion
(D) Who cares about the history of this "mandatory retirement?" It doesn't attack the gap and thus it doesn't matter too much
(E) This might actually hurt the argument. This doesn't seem to support the conclusion because, while the conclusion is saying that a labor shortage would be averted if workers weren't laid off at 65, this answer choice is saying that these workers are still working yet there is still a labor shortage.