How is the flow of energy in an ecosystem characterised?

The correct answer is Option (1) → Unidirectional, from producers to consumers

All organisms depend on producers for food, directly or indirectly. Energy fixed by plants from sunlight passes to herbivores and then to carnivores, and part of it is lost as heat at every step. It never comes back to the sun or to the producers, so the flow is unidirectional.

Option (1) Unidirectional, from producers to consumers — Correct. Energy moves in one direction only: from the sun to producers and then to consumers at higher trophic levels.

Option (2) Bidirectional, between producers and consumers — Incorrect. Energy does not move back from consumers to producers. Nutrients can be reused by producers after decomposition, but energy is not.

Option (3) Circular, within a trophic level — Incorrect. Energy does not go round within a trophic level; it passes up to the next level, with some lost as heat.

Option (4) Absent, as energy is conserved within organisms — Incorrect. Energy is not held inside organisms. It is passed on to a consumer or, when the organism dies, to the detritus food chain, and some is lost as heat at each step.

NCERT Extract:
Energy Flow – “… All organisms are dependent for their food on producers, either directly or indirectly. So you find unidirectional flow of energy from the sun to producers and then to consumers. Is this in keeping with the first law of thermodynamics?”
Energy Flow – “… No energy that is trapped into an organism remains in it for ever. The energy trapped by the producer, hence, is either passed on to a consumer or the organism dies.”