IMO, B.

If there is no evidence of settlement on the island prior to 1650, then that eliminates the possibility that anyone other than the Spanish settlers brought the horses along.

Veritas Prep Official Explanation

1) Horses are not native to the island (so someone had to introduce them)

and

2) Evidence exists that ties horses to the island during the time that the Spanish settlers were there

The question then really is: were the horses already there when the Spanish got there (so someone else introduced them) or did the Spanish bring them there?

Choice E helps to answer that directly: if horses were there before the Spanish got there, then the answer is no, the Spanish did not introduce the horses. Therefore, choice E is correct.

Choices A and C are similar in that they attempt to bring in correlation: other islands, and other species. But since the evidence in the argument is directly tied to the timeline, choice E this the argument head on, and an answer of "yes" to E can completely ruin the argument. With A and C, neither answer (yes or no) can either prove or destroy the argument.

Choice B has a similar problem: sure, the existence of people prior to the Spanish raises the likelihood that someone else may have introduced the horses, but this answer cannot provide direct proof either way: note that the question doesn't even ask "were there other people" but rather "has evidence been found of other people."

Choice D attacks the timeline but in the wrong way: we already have evidence of horses during the Spanish settlement, so establishing whether any people came and stayed after that does not help us date the introduction of horses.
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Santa Clara island is home to a large population of wild horses, but horses are not native to the island. It is known that Spanish settlers colonized Santa Clara island and lived there continuously from 1650 to 1740, and the bones of horses were recently found in an excavation site that can be dated to the early 1700s. Therefore it is likely that the Spanish settlers were the ones who introduced horses to Santa Clara island.

The answer to which of the following questions would be most useful in evaluating the validity of the conclusion drawn above?

A. Did the Spanish settlers introduce horses to any other islands near Santa Clara? Incorrect

other islands are irrelevant

B. Have any excavations found evidence of human settlers who lived on Santa Clara prior to 1650? Incorrect

if no, than Spanish did that
if yes, than not sure whether Spanish did that or not

C. Were any other nonnative species introduced to Santa Clara island by the Spanish settlers? Incorrect

any other species are irrelevant

D. Have any human settlements remained on Santa Clara island since 1740? Incorrect

not related with argument

E. Does Santa Clara island contain any horse bones that can be dated to before 1650? Correct