The answer is below.
Work Step by Step
A resonance structure occurs when the two diagrams are the same, but the only thing that changes is which Oxygen has 6 valence electrons and which Oxygen has the double bond. Recalling this, we find that a) and b) are resonance structures. We find that c) is the one that is equivalent to a), for the only thing that changes is where the valence electrons on the left-most Oxygen are marked. Where we mark the valence electrons on a given atom, however, changes nothing, so these two are equivalent.