Answer
The proton on the right is more acidic.
Work Step by Step
Follow the ARIO order.
1. Atom:
Both hydrogens are connected to a oxygen, so there is no difference.
2. Ressonance:
The proton on the left doesn't make a conjugate base with resonance structures, so the charge is only spread to one oxygen.
In the other side, the proton on the right creates a conjugate base that has resonance structures (There will be a lone pair next to a pi bond), so, the charge will be more delocalized. Therefore, the proton on the right is the more acidic.