The correct answer is option 4 - Flying fish.

Option 1 - Silverfish: Silverfish is not a true fish. It is actually an insect and belongs to the class Insecta.

Option 2 - Jellyfish: Jellyfish, despite its name, is not a true fish. It belongs to the phylum Cnidaria and class Scyphozoa. They are invertebrates and have a gelatinous body structure.

Option 3 - Cuttlefish: Cuttlefish is not a true fish either. it is a marine animal that belongs to the class Cephalopoda along with squids and octopuses. They have a soft body, tentacles, and a unique internal shell.

Option 4 - Flying fish: Flying fish is the only one among the options that is a true fish. They belong to the class Actinopterygii, which is the class of ray-finned fishes. Flying fish have adapted to glide above water, using their wing-like fins to escape predators.

Therefore, out of the given options, only option 4 - Flying fish is a true fish according to the biological system of classification.