Option B is correct because a food chain is a series of organisms feeding on one another at various biotic levels. The context states that autotrophs or producers are at the first trophic level, herbivores or primary consumers at the second, and carnivores or secondary consumers at the third. In option B, grass is the producer (autotroph), goat is the herbivore (primary consumer) that feeds on grass, and human is the carnivore (secondary consumer) that feeds on goat. This forms a proper sequence of organisms where each feeds on the previous one.

Option A has only producers (grass, wheat, mango) with no consumer relationship. Option C has only herbivores (goat, cow, elephant) with no producer or predator-prey sequence. Option D does not form a logical feeding sequence as fish do not naturally feed on grass in terrestrial ecosystems.