Net Primary Productivity (NPP) is given by the formula :

The correct answer is Option (3) → NPP = GPP- Respiration rate

Option (1) NPP = GPP × 100 — Incorrect. Gross primary productivity (GPP) is the rate of production of organic matter in photosynthesis. NPP is always less than GPP, because a part of GPP is used by the plants in respiration, so it cannot be GPP multiplied by 100.

Option (2) NPP = GDP-Secondary productivity — Incorrect. GDP is an economic term; secondary productivity is the rate at which consumers form new organic matter.

Option (3) NPP = GPP- Respiration rate — Correct. Net primary productivity (NPP) is the organic matter left after the plants’ own respiration. So it is gross primary productivity (GPP) minus respiration losses (R), written GPP – R = NPP. This NPP is the biomass available to herbivores and decomposers.

Option (4) NPP = GPP/100 — Incorrect. Dividing gross primary productivity (GPP) by 100 takes away a fixed fraction. What is taken away from GPP is not a fixed fraction but the amount used by plants in respiration.

NCERT Extract:
Productivity – “… Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP). GPP – R = NPP Net primary productivity is the available biomass for the consumption to heterotrophs (herbiviores and decomposers).”