If the solar energy available to a particular ecosystem is 1,000,000…

The correct answer is Option (3) → 1 kJ

Plants capture only a small part of the sunlight falling on them: about half of it is photosynthetically active radiation, and plants capture only 2–10 per cent of that, so roughly 1 per cent of the incident solar energy reaches the producers (first trophic level).

Producers: 1 per cent of 1,000,000 J = 10,000 J. By the 10 per cent law, the second trophic level (herbivores) receives 10 per cent of this = 1,000 J = 1 kJ.

Option (1) 100 kJ — Incorrect. 100 kJ would mean producers capture 10 per cent of sunlight and pass all of it on.

Option (2) 10 kJ — Incorrect. 10 kJ is the energy at the producer level, the first trophic level.

Option (3) 1 kJ — Correct. 1,000,000 J × 1/100 × 1/10 = 1,000 J = 1 kJ.

Option (4) 0.1 kJ — Incorrect. 0.1 kJ is the energy at the third trophic level.

NCERT Extract:
Energy Flow – “… Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation (PAR). We know that plants and photosynthetic bacteria (autotrophs), fix Sun’s radiant energy to make food from simple inorganic materials. Plants capture only 2-10 per cent of the PAR and this small amount of energy sustains the entire living world.”
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.”