First, the blood pressure is higher in the legs than it is in the arms. This is a fact, not a conjecture. It has been verified. For example, if you detect a systolic blood pressure of 60 mm Hg in the brachial artery, then in an average adult you will also observe simultaneously that the femoral artery has a systolic pressure of 70 mm Hg while the carotid has a systolic pressure of 80 mm Hg. Incidentally, these numbers are very low for systolic, and so you often won't see them in a healthy adult. For the purpose of your question, note that 70 mm Hg > 60 mm Hg.
Second, the question implicitly asked what would be a logical reason for this observation. In other words, find a statement that is both true and supportive of the observation. The question makes no claim as to whether the statement (about hydrostatic pressure) is actually the real reason. The real reason for the differential blood pressure findings stem from an eclectic array of factors that interfere with each other in constructive and destructive ways much as several waves of light interfere constuctively and destructively. Your reference to pousillie is an instance of destructive addition, but the constructive addition of the hydrostatic pressure might cancel such an effect out enough to make the pressure greater in the legs anyways. To demonstrate more strongly the muddiness of the question, notice that the hydrostatic explanation is INCONSISTENT with the fact that the 80 mm Hg, which occurs in an artery at a height greater than the femoral artery, is unexpectedly greater than 70 mm Hg. The inconsistency arises due to the fact that the y value towards the carotid artery is negative relative to the reference height at the heart. In contrast, the y term (depth) is positive relative to the same reference height for the femoral artery.
In this question, you should have ruled out the other answer choices. What choice did you pick? Perhaps we could explain your incorrect choice more easily than we could defend AAMC's decision, as is often the case on the MCAT (consider, for example, the Verbal Reasoning section, where often we more easily see why a choice is wrong than we see why another is right).
