Ecological efficiency is:

The correct option is D $\frac{\text{Net primary productivity}}{\text{Gross primary productivity}}\times 100$
Ecological efficiency is the efficiency of transfer of energy from one trophic level to the other.

Ecological efficiency = $\frac{\text{Net primary productivity}}{\text{Gross primary productivity}}\times 100$

Productivity is the rate of biomass production. It can be classified into - (i) gross primary productivity (GPP) and (ii) net primary productivity (NPP).

Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. Whereas, gross primary productivity minus respiration losses (R) is the net primary productivity (NPP) i.e., GPP – R = NPP.