Option A: 2 trophic levels
A food chain with fewer trophic levels wastes less energy. Since only 10% of energy passes to the next level, a 2-trophic-level chain transfers the maximum energy to the consumer.
The 10% energy law states only 10% of energy is transferred at each trophic level; 90% is lost as heat. So, the fewer the trophic levels, the more energy is available. A 2-trophic-level chain (producer → consumer) is most energy-efficient. This is a favourite MCQ based on energy flow concepts.