It's a badly worded question, but the answer that is being looked for would be "C".
A. increases a solution’s pH
No, an acid increases a solution's pH provided the solution is an aqueous one. If it's a non aqueous solution then it will not have a pH in the first place!
B. produces hydroxide ions
Not sure about the the term "produces", but no, hydroxide ions can result from, for example, the dissolution of a metal hydroxide in water, but not from an acid.
C. donates a proton
Yes, but thats only if you're considering a Brønsted - Lowry acid.
D. acts as a hydrogen acceptor.
Guessing the question is referring to protons, i.e. hydrogen iions, but no, a substance acting as a proton acceptor would be a Brønsted - Lowry base.