In a grazing food chain, what part of the energy at one trophic level…

The correct answer is Option (2) → 10 per cent

As energy passes up a grazing food chain, most of it is used up or lost as heat at each step. Only about 10 per cent of the energy at one trophic level reaches the next higher level. This is the 10 per cent law, and it is the reason why a grazing food chain cannot have many trophic levels.

For example, if the producers hold 10,000 J, the herbivores get about 1000 J and the carnivores that eat them about 100 J.

NCERT Extract:
Energy Flow – “The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10 per cent law – only 10 per cent of the energy is transferred to each trophic level from the lower trophic level. In nature, it is possible to have so many levels – producer, herbivore, primary carnivore, secondary carnivore in the grazing food chain (Figure 12.3).”