Flying fish is the true fish according to the biological system of classification. Let's understand why this is the correct answer by examining the characteristics of each option:

a) Silverfish: Silverfish, scientifically known as Lepisma saccharina, is not a fish. It is actually an insect belonging to the order Zygentoma. Silverfish are small, wingless insects that are often found in dark and damp places, such as basements and bathrooms. They have a silver-gray color and a fish-like appearance, but they are not true fish.

b) Jellyfish: Jellyfish, scientifically known as Medusozoa, are not true fish either. They are marine animals that belong to the phylum Cnidaria. Jellyfish have a gelatinous body and tentacles that they use to capture prey. Although they are aquatic creatures, they are not classified as fish.

c) Cuttlefish: Cuttlefish, scientifically known as Sepiida, are marine animals that belong to the class Cephalopoda. They are closely related to squids and octopuses. Cuttlefish have a unique internal shell called a cuttlebone, which helps them control their buoyancy. While they are often referred to as "fish," they are not true fish but rather mollusks.

d) Flying fish: Flying fish, scientifically known as Exocoetidae, are indeed true fish. They belong to the family Exocoetidae, which includes around 64 species. Flying fish have adapted to be able to glide above the water surface by extending their large pectoral fins. This unique ability allows them to escape predators and cover long distances. Despite their ability to fly for short periods, they are still considered fish as they possess the characteristic features of fish, such as gills for respiration and scales for protection.

In conclusion, the true fish according to the biological system of classification is the flying fish (option D). It is important to note the distinction between true fish and other organisms that may have fish-like appearances or live in aquatic environments.