(c) Plants → Man
This chain has only two trophic levels, so energy loss is minimum (only 10% is lost once). Fewer steps mean more energy is available at the final consumer level.
Source: Chapter 13, Section 13.1.1
The 10% energy law states that only 10% of energy transfers to the next trophic level. The shorter the food chain, the less energy is wasted. Plants → Man has just 2 trophic levels (one transfer), retaining the most energy, making it the most efficient. Examiners expect you to state the reason (fewer trophic levels = less energy loss), not just the option.