Which of the following statements is true about ecosystem?

The correct answer is C. Energy moves from producers to consumers, and roughly 90 percent is lost at each step as heat. So in the usual pyramid of numbers, producers (plants and algae) outnumber primary consumers (herbivores), which outnumber secondary consumers (carnivores that eat herbivores).

• A is wrong. Primary consumers are the most directly dependent on producers, because they feed on them straight away and have no other basic food source.
• B is wrong. Herbivores are fewer than the plants that support them. The pyramid of numbers is upright in grasslands and most aquatic systems.
• D is wrong. Secondary consumers are not the largest or most powerful group in number. Top carnivores are the fewest and receive the least energy.

Concept: trophic levels and ecological pyramids. This links to Environment and Ecology in Prelims and to Mains questions on why top predators are vulnerable and how food chain disruption affects biodiversity.

Trap: Choosing A because herbivores seem loosely 'dependent' on plants, when the question is about relative numbers and structural dependence.