I probably would have picked "C" as well, but can see what they were trying to get at with their selected answer. I don't agree with it, but I can see the point. It's an academic answer that probably aligns with something in the PMBOK, but that doesn't mean it's either a great question or a great answer.
Ivory Tower Answers
Most tests, and especially most prep tests, often contain ambiguous questions or answers that are wrong from a practical perspective. This isn't necessarily a deliberate practice to hone your test-taking skills; it's often just a side-effect of having to select questions from a pool that is not allowed to precisely mirror test questions that are often protected by NDAs or copyright.
Even on the actual tests, there are often questions which are iffy because the selected answers reflect an academic (rather than practical) bent on the part of the test creators. This is a pervasive problem when the test writers are academics rather than practitioners, which is why most good tests are normed on the responses.
As a singular example, when I sat for the CISSP many years ago, I remember a particularly horrible question about virus security. The question was something along the lines of "What's the best way to prevent introduction of a virus onto a computer system?" The use of an anti-virus system was the option that any real-world security practitioner would select, but I knew that the test writers expected "certify all media" to be the correct answer for that particular test. Unless you understood that the test was written by academics who focus on abstruse government information assurance practices, you were bound to get the answer "wrong."
Unless you fail an actual exam, I wouldn't lose any sleep over questions like this. Depending on the exam, you also have a few other strategies to consider.
- Give the answer you think the test writers are looking for, rather than the correct answer.
- If your test allows it, select an answer and then comment on why you think the "correct" answer is wrong or why you think a different answer would be more correct. Especially on experimental questions that are included for norming, this can be a very effective strategy.
- Pick the answer you genuinely feel is correct, and rely on norming (e.g. "grading on a curve") to discount ambiguous or commonly-missed questions.
Self-Evaluation and Domain Mastery
Finally, remember that sample tests are just samples, and are rarely drawn from the actual test material. They're really just there to help you self-evaluate your confidence level in the material. Sometimes, knowing that a sample test contains invalid questions or iffy answers is as good a way to measure your domain knowledge as anything else.
By all means, if you find questions that merit additional study on your part, do that. However, sometimes it's more important to know that your answer is the one an employer would pay you to perform, regardless of what the answer key for a sample test says.