Q.Which one of the following has the largest population in a food chain?

Biomass pyramids are inverted in pond/aquatic ecosystems (small, fast-turnover producers support a larger consumer biomass) but upright in most terrestrial ecosystems -- so the pyramid of biomass is not literally always inverted, unlike the pyramid of energy, which is always upright.

A pyramid of biomass shows the standing crop (total dry weight) of organisms present at each trophic level at a given point in time. Whether this pyramid is upright or inverted depends on the ecosystem:

1. Terrestrial ecosystems (e.g. forest, grassland): producers (trees, grasses) are large, long-lived organisms that accumulate a very large standing biomass, decreasing at successive trophic levels -- giving the normal, upright pyramid shape.

2. Pond/aquatic ecosystems: the producers are microscopic phytoplankton (algae), which are extremely small individually and have very short lifespans, but reproduce and are consumed very rapidly (high turnover rate). At any single moment, their standing biomass is therefore small, even though their rate of production (productivity) over time may be high. The primary consumers (zooplankton) and the fish that feed on them are larger-bodied and longer-lived, so they accumulate a greater standing biomass than the producers supporting them at that instant. This makes the biomass pyramid of a pond ecosystem inverted (small producer biomass at the base, larger consumer biomass above).

So the correct explanation is that the biomass pyramid is inverted only in certain aquatic ecosystems like ponds -- due to the mismatch between a producer's small standing biomass and its high turnover rate -- while it remains upright in most terrestrial ecosystems; the pyramid that is genuinely always upright, in every ecosystem, is the pyramid of energy.

An energy pyramid always has a broad base and narrow apex — energy shrinks at every trophic level.

An energy (Eltonian) pyramid is a graphical representation of the amount of energy present/flowing through each successive trophic level of a food chain (producers, primary consumers, secondary consumers, and so on) over a fixed period of time and area, usually expressed as kcal/m2/year. According to the ten-percent law (Lindeman's law), only about 10% of the energy received by one trophic level is transferred to and stored in the next level; the remaining ~90% is lost as heat during respiration, in undigested/unused food, and other metabolic losses. Because energy is progressively lost at each transfer, the pyramid of energy is always upright — broad at the producer base and narrowing towards the top carnivores — and, unlike pyramids of numbers or biomass, it is never inverted, in any ecosystem.