The rate of alpha halogenation is independent of the concentration (or identity) of the halogen. The rate depends only on the concentration of aldehyde or ketone. What does that tell you? The best conclusion must be that halogen does not participate in the rate determining step of the reaction. What is the rate determining step? The answer is enolization.

Can you think of some 'wrong' answers a test-writer might think up to put with correct ones like these above within an MCAT multiple choice question? Here's one. Maybe they would try to trick you with some half remembered idea from enzyme kinetics, even though there is no enzyme involved, like trying to get you to agree that halogen must be present in greater than saturating concentration. Try not to think like the test-writers too much! It will change your view of human nature.