An octopus belongs to the Phylum Mollusca, making option 4 the correct answer.

Option 1 suggests that an octopus is an arthropod. Arthropods are a group of invertebrates which includes insects, arachnids, and crustaceans. They are characterized by their jointed limbs and cuticle made of chitin, which are not features seen in octopuses.

Option 2 suggests that octopus is an echinoderm. Echinoderms include species like starfish and sea urchins. They are usually characterized by radial symmetry, generally have a spiny skin or shell, and are exclusively marine species. But octopuses do not share these characteristics.

Option 3 suggests that the octopus is a hemichordate. Hemichordates are a phylum of marine invertebrates that`s closely related to echinoderms and chordates. They include species like acorn worms and graptolites. Octopuses do not fall into this category.

Hence, summarizing, Octopuses are molluscs that belong to the class Cephalopoda, which also includes squid and cuttlefish. They`re characterized by a soft body and absence of external