The word sharpen is a word that leads you to believe you are talking about knives. It has nothing to do with knives.
A: implies the same thing. It just isn't so.
B: dull is also a term that is associated with knives. In this case, it is factually false (you want to "sharpen" something, not make it dull) and it is not to be associated with a knife. Don't choose B.
Sharpen your brain means that you become more agile in your thinking. None of the choices really says that. So you have to pick something that is the "best" choice of 4 very poor ones. I suppose C is the answer, but I'm holding my nose as I tell you this. (C as an answer stinks).
D is 1/2 true but you have to associate it with thinking. I don't think there is a really good answer, but of those there, I would pick C.
