Net primary productivity can be defined as:

The correct answer is Option (3) → Available biomass for the consumption to heterotrophs

Gross primary productivity (GPP) is the total organic matter that plants make by photosynthesis. Plants use part of it in respiration (R). What is left is the net primary productivity (NPP).

(1) Available biomass for the consumption to producers — Incorrect. NPP is meant for the consumers, not for the producers who made it.

(2) Rate of formation of new organic matter by consumers — Incorrect. This is secondary productivity.

(3) Available biomass for the consumption to heterotrophs — Correct. Herbivores and decomposers feed on this biomass.

(4) Amount of biomass produced by plants during photosynthesis — Incorrect. This is GPP. NPP is what remains after the respiration loss.

NCERT Extract:
Productivity – “… 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). Secondary productivity is defined as the rate of formation of new organic matter by consumers.”