Why is o-halophenol more acidic than m-halophenol?
It's because the halogen is highly electronegative. Therefore, the closer the halogen is to the alkoxide, the more it helps in stabilizing its negative charge after it's been deprotonated. Conversely, the further away the halogen is, the less it is able to help stabilize the negative charge. The more stable the conjugate base, the stronger the acid.

This question is basically testing induction, which is covered pretty well here:

http://orgomadesimple.com/organic-chemistry-acids-and-bases/