Coincidence detector, as the name suggests, is something which will detect coincidence in your inputs, i.e, it will give a high output when your inputs coincide with each other.
Such a property is shown only by X-NOR gate, i.e. it gives a high output if both inputs are either high only or low only.
So, X-NOR is the answer.
P.S - Some books even show AND as the coincidence detector, but it is not as it doesn't show a high input when both inputs are low.