(C) Decrease in energy at higher levels
The loss of energy at each trophic level is so great that very little usable energy remains after four trophic levels, limiting food chain length.
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The textbook (Chapter 13, "What you have learnt") explicitly states: "There is a loss of energy as we go from one trophic level to the next, this limits the number of trophic levels in a food-chain." Only ~10% of energy passes to the next level, so after 3–4 levels, usable energy is negligible. Examiners expect you to pick (C) and may ask for a one-line justification.
Source: Chapter 13, Section 13.1.1 & What You Have Learnt