Acidity increases with increasing stability of the conjugate base (there are exceptions to this, but I doubt there will be any on a standardized test). The more freedom electrons have to move around a structure, the more stable that structure is. For this reason, resonance imparts (significant) stability. Compare ethanol and phenol. Ethanol has a pKa of about 16, while phenol has a pKa of about 10. This means phenol is 10^6 times a stronger acid than ethanol. This is because the phenoxide ion has 4 resonance structures over which to spread it's electron density, while ethoxide's negative charge is stuck on the oxygen.
