Arrange the following ecosystems in increasing order of mean NPP :
A. Tropical deciduous forest
B. Temperate coniferous forest
C. Tropical rainforest
D. Temperate deciduous forest

The correct option is D B < D < A < C
Net primary productivity (NPP) is the biomass or storage of energy by green plants. It is equal to the gross primary productivity minus loss due to respiration.
The productivity generally increases from polar regions toward the tropics, because of the increasing sunlight and temperature.
So the NPP will be the lowest in the coniferous forest where the termperature is cold and will be highest in tropical rainforest where the temperature will be humid and warm.
The ascending order of NPP will be: Temperate coniferous forest> Temperate deciduous forest > Tropical deciduous forest > Tropical rainforest.