The correct answer is (a) Decrease in energy at higher trophic levels.
Explanation: As energy moves up a food chain, a significant portion of it is lost as heat at each level, meaning there is less energy available for organisms at higher trophic levels, limiting the number of levels that can exist.
Why other options are incorrect:
(b) Sufficient food supply:
While a lack of food can limit population sizes, it doesn’t directly limit the number of trophic levels in a food chain. The number of levels is primarily constrained by energy loss, not the overall amount of food available.
(c) Polluted air:
Polluted air can harm organisms at various trophic levels, but it doesn’t fundamentally limit the number of trophic levels.
(d) Water:
Water is essential for life, but its availability doesn’t directly determine the number of trophic levels in a food chain. The energy limitation factor is the key determinant.