Answer: (A)
An assumption is something that HAS to be true for the argument to hold.
Consider the argument and how it is made:
1. Many R (complex repair jobs) have to be reworked.
2. Reworked jobs are satisfactory, but initial jobs are not.
3. Therefore, mechanic competence is not the problem, something else must be (viz. concentration rather than skill)
The conclusion of the argument is hidden in the second point. The author concludes that reworks are NOT because of a lack of skill. Now, what could make this argument fall apart? What if there are two different sets of people that carry out initial repair vs. reworks? Hmm... Indeed, if that's the case, the conclusion cannot be drawn. So, for the author to reliable draw the conclusion, this must be FALSE - the set of workers MUST be the same.
Let's see what the options suggest (see inline):
Huajun
A significant number of complex repair jobs carried out by Ace Repairs have to be reworked under the company's warranty. The reworked jobs are invariably satisfactory. When initial repairs are inadequate, therefore, it is not because the mechanics lack competence; rather, there is clearly a level of focused concentration that complex repairs require that is elicited more reliably by rework jobs than by first-time jobs.
The argument above assumes which of the following?
[A] There is no systematic difference in membership between the group of mechanics who do first-time jobs and the group of those who do rework jobs. =>
If this is FALSE, i.e. if there IS a difference between the workers, the argument falls apart. Thus, above statement is necessary for the argument to hold and is therefore an assumption.
[B] There is no company that successfully competes with Ace Repairs for complex repair jobs. => out of scope. other companies are irrelevant.
[C] Ace Repairs' warranty is good on first-time jobs but does not cover rework jobs. => suppose it does cover rework jobs. does that have any bearing on the conclusion about worker skill? no. thus, negating the statement does not affect the argument. therefore, this is not something on which the argument DEPENDS.
[D] Ace Repairs does not in any way penalize mechanics who have worked on complex repair jobs that later had to be reworked. =>irrelevant. it doesn't matter if they do penalize the mechanics. negating this does not have any bearing on mechanic skillset.
[E] There is no category of repair jobs in which Ace Repairs invariably carries out first-time jobs satisfactorily. => there might be some such categories. a significant number does not mean "all". also, this is not related to the skill of mechanics.