Explanation

Reading the question: The question asks us to strengthen, so we will first attack. There are a number of points we can attack, but reasoning involving percentages tends to be erroneous, so we will focus there. We can filter for "not confusing numbers and percentages" as a strengthener.

Applying the filter: choices (A), (B), and (E) all fail to clarify the distinction between absolute number and percentage, so they fail to pass our filter. Choices (C) and (D) are close to the filter.

Logical proof: we can evaluate choice (C) by the negation test. What if the number of international travelers had not increased over the past three years? What if it had plummeted? That is in fact not accounted for by the prompt, and it would crush the argument. The percentage of the pie who visit Kokua is larger, but the pie is much smaller, so the actual number of visitors to Kokua could have greatly decreased. By ruling out that possibility, the non-negated (C) fixes a major flaw in the argument and is therefore a strengthener. Choice (D), which we liked at first, is still vulnerable to (C), as is the argument as a whole. The correct answer is (C).