The correct option is A Mollusca
Mollusca composes the 2nd largest phylum of animal kingdom after arthropods, which is the largest phylum in the animal kingdom. Around 85,000 species of molluscs are recognised. Molluscs is the largest marine phylum, comprising about 23% of all the named marine organisms. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. Hence, option C is correct.