There will still be some hydrogen bonding, but that won't necessarily increase the melting point. The issue is whether or not any hydrogen bonding between 4-AB and camphoric will be stronger or weaker than the h-bonding of just pure 4-AB. Generally pure substances pack together better and bond more tightly, so while the camphoric acid will hydrogen bond with 4-AB, this bonding won't be as strong as if your sample contained only 4-AB and will thus lower the melting point.
I didn't even say that, a pure compound with impurities always has BP elevation, MP depression, VP depression. A and D don't make sense, and a "small percentage" isn't numerical, so B.