Final answer:
Cnidocytes, containing nematocysts that release toxins, are found in the phylum Cnidaria, which includes marine organisms like jellyfish and corals.
Step-by-step explanation:
Cnidocytes are specialized cells unique to the phylum Cnidaria, which includes diverse aquatic organisms such as jellyfish, corals, and sea anemones. These cells are equipped with structures called nematocysts — a type of organelle that can deliver toxins to prey and predators, thereby assisting in defense and feeding. Cnidarians use these stinging cells to paralyze or capture their prey with the help of the fast-firing mechanism triggered by the cnidocils, hairlike projections on the cells.
When considering where cnidocytes are found, the correct answer is d) phylum Cnidaria. This group is characterized by their radial symmetry, diploblastic tissue organization, and predominantly marine habitat. Unlike phylum Porifera (sponges) that lack true tissues, and other phyla like Nemertea (ribbon worms), Nematoda (roundworms), and Annelida (segmented worms), cnidarians uniquely possess cnidocytes for their survival in their aquatic environments.