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Let think about a cation in solution. It is going to have its own shell of hydration because it is going to be surrounded by water molecules (another good question would be to think about how the water molecules would be pointed around the cation). However, the more important idea to understand is that, in order to enter the channel, the cation MUST SHED its shell of hydration and for that to happen, it must be MORE attracted to whatever is on the inside of the channel. In this case, the cation is something that has a positive charge, and thus, for it to be attracted to something within the channel, the channel ought to be negatively charged. The amino acids that tend to be negatively charged are those that are acidic, which is why B is the correct answer.