Let’s understand the details of the argument.

1) A significant number of complex repair jobs carried out by Ace Repairs have to be reworked under the company’s warranty.
2) The reworked jobs have always been satisfactory.
3) the initial repairs are not enough,

Conclusion- 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 question is to find the assumption.

(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.
Let’s negate option A to see if it will impact our conclusion.
The negation of A gives us- There is a systematic difference between the group of mechanics who do first-time jobs and the group of mechanics who do rework jobs or that - more competent mechanics work on rework jobs and less competent mechanics work on first-time jobs.

This would mean that it is because of the competency of the mechanics that the rework jobs are always satisfactory and the first-time jobs are always brought for rework- This would negate the conclusion. Hence A is the assumption on which the argument is based on.

(B) There is no company that successfully competes with Ace Repairs for complex repair jobs.

Even if there were a company that successfully competed with Ace Repairs for complex repair jobs, it would still not affect the conclusion. Eliminate B

(C) Ace Repairs’ warranty is good on first-time jobs but does not cover rework jobs.

So? Irrelevant. Eliminate C

(D) Ace Repairs does not in any way penalize mechanics who have worked on complex repair jobs that later had to be reworked.

Even if it did, it will not impact the conclusion. Eliminate D


(E) There is no category of repair jobs in which Ace Repairs invariably carries out first-time jobs satisfactorily.

Let’s try and negate this

There is a category of repair jobs in which Ace Repairs invariably carries out first-time jobs satisfactorily.

How much does the category constitute? If it constitutes a significant size, it would go against the premise and not the conclusion. Eliminate E