I haven't started studying, well, re-studying orgo yet, still working through gen chem. But having looked at the question, I assumed immediately the answer was C (number 3).
The most acidic Hydrogen IIRC, is the one that can most easily leave/be plucked away. Often it's the one where the most combination of bonds can be formed (resonance?) after the Hydrogen is taken and its electrons need a place to go. Terrible explanation I know... I'm just too rusty.
In our molecule though, you can see that if number 3 is taken, his electrons can go into a double bond to the left or the right, and then the double bond to either of the Oxygens can go up to the oxygen as a lone pair. So this is the only one with so much flexibility so to speak with how the electrons re-arrange.
Sorry, I'm sure you'll get a better explanation very soon