NPP (Net Primary Productivity) is highest for

Net Primary Productivity (NPP) is the rate at which primary producers store energy as biomass, calculated as Gross Primary Productivity (GPP) minus the energy lost to respiration.

Among the given options, tropical forests have the highest total Net Primary Productivity. They experience ideal climatic conditions for plant growth—year-round high temperatures, abundant sunlight, and heavy rainfall—which allow for continuous photosynthesis without a dormant season.

While specialized ecosystems like reefs and swamps exhibit extremely high NPP per unit area, their total global coverage is very small. Consequently, their overall contribution to the Earth's net primary production is only a fraction of that of tropical forests. Because standard examinations evaluate overall global ecosystem contribution unless "per unit area" is explicitly specified, tropical forests are considered to have the highest NPP.