The correct exception a state is allowed to make to the comity clause is reasonable discrimination.
The comity clause, which is found in the Full Faith and Credit Clause of the U.S. Constitution, generally requires states to recognize the public acts, records, and judicial proceedings of other states. However, states may make reasonable distinctions or discriminations in certain circumstances, particularly in areas such as taxation, social services, and some legal regulations, as long as they do not violate constitutional rights.
The other options listed (defense of marriage, extradition, privilege) are either related to specific legal contexts or do not directly pertain to exceptions under the comity clause.