Answer:
The wrong pairing is class Gastropoda; mussels.
Step-by-step explanation:
The first option is correct because the mussels don't belong in the class Gastropoda, but they are part of the class Bivalvia.
The second option is not correct because the oysters, just as the mussels, are part of the class Bivalvia, and are among the most numerous in it.
The third option is not correct because the octopuses are one of the leading members of the class Cephalopoda, being the largest on average, but also having a reputation or being among the most intelligent animals on our planet.
The fourth option is not correct because the chitons are a group of shelled marine animals that belong to the class Polyplacophora, and there are 940 species in existence.