NPP (Net Primary Productivity) is highest for

Net Primary Productivity (NPP) measures the energy or organic matter stored at the primary producer level after accounting for respiratory losses [1]. While terrestrial ecosystems like tropical rainforests are highly productive due to consistent high temperatures and humidity [3], aquatic and transitional ecosystems often exhibit higher NPP per unit area. Specifically, coral reefs are recognized as having the highest rates of primary productivity among all biomes, often reaching approximately 2,500 grams of biomass per square meter per year. Swamps and marshes also show exceptionally high NPP, sometimes rivaling or slightly exceeding tropical forests, but coral reefs consistently peak in productivity comparisons. Tropical rainforests, while having the highest total global terrestrial NPP, are surpassed on a per-unit-area basis by these specialized aquatic systems. Therefore, reefs represent the ecosystem with the highest NPP among the provided options.

Sources

1. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > ECOLOgICAL PrODuCTIvITy. > p. 33
2. [3] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 3: Terrestrial Ecosystems > 3.2.5. Tropical rain forests: > p. 25