This is an assumption question. We're looking for a necessary. Without which the argument and its conclusion would collapse.
So, the argumentation is that injekction printers are more reliable than laser printers. Therefore, the conclusion is that firms should choose inkjet printers.
The necessary assumption is B. Without B, the conclusion in the argument would collapse.
The answers B introduces a new factor that is necessary for the conclusion to work. B is a necessary assumption for the conclusion to be valid. The explanation is that, since we are comparing two different groups of printers, we have to compare them under the same conditions. B explicitly states that the print volume of both groups of printers is approximately the same.
If that were not be the case (Negation Test) - for example, if laser printers were used 1,000 times more often - it would logically imply that they would require more servicing. The argument and therefore the conclusion, would collapse.
In other words, we must compare both groups under the same conditions and this is ensured by assumption B.
Now let's see why the other answers are wrong (since the elimination method is sometimes more useful than directly identifying the correct answer).
A) is WRONG. If we negate B (Inkjet printers produce lower print quality), the argument would still hold. In other words, we would still recommend to use inkjet printers EVEN IF they produced lower print quality, since reliability is the most important factor in the argument.
C) is WRONG. It doesn't matter how many printers were purchased.
We can negate C) (for example, some offices acquired more laser printers than inkjet printers), and the argument would still hold. The recommendation to buy inkjet printers remains valid.
(For some people, it doesn't make much sense to negate a positive answer choice (C). They only apply a Negation Test to negative answer choices such as A),B), D), E))
D) is WRONG. Even if laser printers required longer downtown for each service visit, the argument and its conclusion would still hold. In fact, this would even strengthen the recommendation to use inkjet printers.
E) is WRONG, BUT it is the MOST TEMPTING ANSWER CHOICE!!! (IT'S A TRAP!!) Let's see why?
It means that inkjet printers in the survey were not significantly newer than the laser printers (or, more simply, that the age of all the printers was approximately the same). Although this strengthens the conclusion, it is NOT a necessary assumption. The argument can still stay without it.
NOTE on E): Yes, it introduces a new factor that should be considered (as in B), but the argument with its conclusion (firms should buy inkjet printers) remains valid even without this condition, it even strengthens the conclusion! (BUT without B, the argument collapses.)
Remark: We don't need to apply the Negation Test to every answer choice (because of time constraints on the GMAT). However, it is especially useful when we are deciding between two plausible answer choices.