A peacock eats a snake, the snake eats a frog and the frog eats insec…

The correct answer is Option (4) → At the apex of the ecological pyramid

Each step of a food chain is a trophic level. Here the chain is: green plants (producers) → insects (primary consumers) → frog (secondary consumer) → snake (tertiary consumer) → peacock. The peacock eats the snake and nothing in the chain eats the peacock, so it is the top-level consumer. In an ecological pyramid the producers form the base and the top consumer forms the apex.

Option (1) Primary producer — Incorrect. The primary producers are the green plants, which fix the sun’s energy. The peacock is an animal that eats other animals.

Option (2) Secondary consumer — Incorrect. The secondary consumer is the animal that eats the herbivore. Here that is the frog, which eats insects, not the peacock.

Option (3) Decomposer — Incorrect. Decomposers, mainly fungi and bacteria, break down dead organic matter in the detritus food chain. The peacock is a predator in the grazing food chain.

Option (4) At the apex of the ecological pyramid — Correct. The peacock is the last link of the chain, the top consumer, so it sits at the apex of the ecological pyramid.

NCERT Extract:
Ecological Pyramids – “… The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer. The three types of ecological pyramids that are usually studied are (a) pyramid of number; (b) pyramid of biomass and (c) pyramid of energy.”
Energy Flow – “… All animals depend on plants (directly or indirectly) for their food needs. They are hence called consumers and also heterotrophs. If they feed on the producers, the plants, they are called primary consumers, and if the animals eat other animals which in turn eat the plants (or their produce) they are called secondary consumers. Likewise, you could have tertiary consumers too.”