The correct option is B Cestoda
Gut parasite belongs to Cestoda. As adults, they are obligate parasites in the gut of vertebrates. Cestodes have no digestive system and absorb nutrients from the host's alimentary tract through their specialized neodermal cuticle or tegument through which gas exchange also takes place. The tegument also protects the parasite from the host's digestive enzymes and allows it to transfer molecules back to the host. So, the correct answer is 'Cestoda'.