Which of the following pairs of organisms and trophic level is correc…

The correct answer is Option (3) → Molluscs in an aquatic ecosystem – Second trophic level

Organisms are placed in trophic levels by the source of their food. Producers are at the first level and herbivores (primary consumers) at the second. Carnivores feeding on herbivores are at the third, and top carnivores at the fourth.

Option (1) Zooplankton and grasshopper – Third trophic level — Incorrect. Zooplankton and the grasshopper feed on producers, so they are herbivores at the second trophic level.

Option (2) Man and lion – Third trophic level — Incorrect. Man and lion are shown as top carnivores (tertiary consumers) at the fourth trophic level.

Option (3) Molluscs in an aquatic ecosystem – Second trophic level — Correct. Molluscs are a common herbivore of aquatic ecosystems. Being primary consumers, they belong to the second trophic level.

Option (4) Phytoplankton and trees – Second trophic level — Incorrect. Phytoplankton and trees are producers, so they are at the first trophic level.

Note: The paragraphs on trophic levels stop at the third level and never name man or lion. Those names live on the right-hand side of the boxes in Figure 12.2, along with zooplankton, grasshopper, phytoplankton and trees. A flow chart with three columns is three times the reading, and three times the marks.

NCERT Extract:
Energy Flow – “… Obviously the primary consumers will be herbivores. Some common herbivores are insects, birds and mammals in terrestrial ecosystem and molluscs in aquatic ecosystem.”
Energy Flow – “… Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level. Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third (Figure 12.2).”

Figure 12.2 Diagrammatic representation of trophic levels in an ecosystem