Which of these sea creatures is classified as a fish?

A seahorse is any of 46 species of small marine fish in the genus 'Hippocampus'. 'Hippocampus' comes from the Ancient Greek 'hippókampos', itself from 'híppos' meaning 'horse' and 'kámpos' meaning 'sea monster' or 'sea animal'. Having a head and neck suggestive of a horse, seahorses also feature segmented bony armour, an upright posture and a curled prehensile tail. Along with the pipefishes and seadragons ('Phycodurus' and 'Phyllopteryx') they form the family 'Syngnathidae'.

Seahorses range in size from 1.5 to 35.5 cm (5⁄8 to 14 in). Although they are bony fish, they do not have scales, but rather thin skin stretched over a series of bony plates, which are arranged in rings throughout their bodies. Each species has a distinct number of rings. The armor of bony plates also protects them against predators, and because of this outer skeleton, they no longer have ribs. Seahorses swim upright, propelling themselves using the dorsal fin, another characteristic not shared by their close pipefish relatives, which swim horizontally.

Seahorses are mainly found in shallow tropical and temperate salt water throughout the world.

In regard to the other options: Scallop is a common name that encompasses various species of marine bivalve mollusks in the taxonomic family 'Pectinidae'. An octopus is a soft-bodied, eight-limbed mollusc of the order 'Octopoda'. The Portuguese man o' war ('Physalia physalis'), is a marine 'hydrozoan' found in the Atlantic Ocean and the Indian Ocean.