Trophic levels are formed by -

A trophic level is the functional level or position an organism occupies in a food chain based on its source of nutrition or food. These levels are formed by organisms linked in a food chain through the sequential transfer of energy.

• First Trophic Level: Primary producers (autotrophs like green plants).
• Second Trophic Level: Primary consumers (herbivores).
• Third Trophic Level: Secondary consumers (carnivores).
• Fourth Trophic Level: Tertiary consumers (top carnivores).

Options A, B, and D are incorrect because they represent only specific segments of the ecosystem. Trophic levels encompass the entire hierarchy of life forms, from producers to various levels of consumers, interacting within the energy flow of an ecosystem.