Which of the following is a true fish?
(A) Flying fish
(B) Cuttlefish
(C) Silverfish
(D) Jellyfish

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Hint: A fish belonging to the Exocoetidae family is a real or true fish. They have long-winged fins that enable gliding for a wide distance, and also they have streamlined bodies with scales.

Complete answer:
> Exocoetus is a flying fish and belongs to the family of marine fish in the class Actinopterygii. It is a true fish, since it rationalizes the body with scales and breathes through gills.

> Cuttlefish are the molluscs of the Sepiida order, which belongs to cephalopod class. They are not considered as true fish because they don't have bones, heart and brain. Fresh water is used as a nutrient source rather than blood.

> Silver fish is a little wingless insect in the order of Thysanura or Lepisma. Thus these fishes cannot come under the category of true fish. Also it doesn’t have gills and other essential features which true fishes have.

> Jellyfish in phylum Cnidaria are the predominant non-polyp species. Jellyfish are not classified as fish as they do not comply with the fish criteria. Gills, fins and vertebra are some of the basic requirements which an organism must fulfil if it is to be called a true fish and these features are not found in a jellyfish, hence it is not a true fish.

Hence, The correct answer is option (A).

Additional information:
True fishes have the backbone and the fins. Most of them also breathe with gills and have scales that cover their bodies.
Although they cannot fly the same way a bird does, flying fish can make strong, self-propelled leaps out of the water where their long wing-like fins allow for long periods of gliding over the surface of the water. The primary explanation for this action is thought to be the protection from predators.

Note: In contrast to mammals, fish are cold-blooded (ectothermic). This means that they do not maintain a constant internal body temperature; instead, their temperature is strongly influenced by their surroundings. Here in flying fish there is presence of scales on the body and respiration by gills are the main feature of them, thus categorizing them as a true fish.