At a wildlife sanctuary, two methods are usually employed to tranquili

It's not clear what the vague word "effective" means in the conclusion, and since sprays have some advantages and injections have some advantages, the assumption behind the argument is that the advantages of injections (injections take less time to tranquilize and animal) are an important part of what makes a method "more effective".

But no answer choice says that. Not one of the answers is an assumption here. Most of the answers are clearly wrong -- C is not about treating injured animals, the only animals the stem discusses, so is irrelevant; D has nothing to do with the comparison between sprays and injections so is irrelevant; we don't care by what mechanism injections are effective, only that they are effective, so E is irrelevant.

It's impossible to evaluate A, because there's no way to know what "more effective" means. If "more effective" means "remains tranquilized longer", then A is certainly an assumption (but then we didn't need to read any of the evidence in the stem). If "more effective" means something else, though, then A is not an assumption.

B, which is listed as the OA, is clearly not an assumption. If you insert B in the argument, that clearly makes the argument valid -- if we know that effectiveness and tranquilization time are inversely proportional, then the method that takes the least time is automatically, by definition, the most effective. But that's not what an assumption is. An assumption is something that must be true for the argument to hold. We don't need to assume that there exists a specific type of proportionality between effectiveness and tranquilization time for this argument to be valid.

So there is no good answer to this question.