Which atom mostly limits the primary productivity?
A) P
B) O
C) N
D) $CO_2$

Hint: Primary productivity in ecological terms is the process of synthesis of organic compounds from the atmosphere or aqueous carbon dioxide. It principally occurs through photosynthesis but sometimes it also occurs through the chemosynthesis.

Complete answer:
Solar energy is the basic requirement for any ecosystem to function and sustain either its grassland ecosystem or aquatic ecosystem. Primary production is defined as the amount of biomass or organic matter produced (as a food) per unit area over a period of time by plants during photosynthesis. It is expressed in terms of weight as $g^{-2}$ or energy as $k cal m^{-2}$.
The rate of biomass production is called productivity expressed in terms of $g^{-2}$ $yr{^-1}$ or $kcal m^{-2} yr^{-1}$ to compare the productivity of different ecosystems. It mainly utilizes carbon dioxide, water, and sunlight. Thus the carbon acts as a limiting atom in the primary productivity.

Additional information:Primary productivity can be divided into gross primary productivity (GPP) and net primary productivity (NPP). GPP is the rate of production of organic matter during the process of photosynthesis while NPP is the available biomass for the consumption of heterotrophs like herbivores and decomposers.
$\Rightarrow$ GPP-R=NPP
Where GPP= Gross primary productivity; R= respiration; NPP= net primary productivity
A considerable amount of GPP is utilized by plants during the process of respiration.

The correct option is D i.e. C (carbon dioxide).

Note:Primary productivity depends on the plant species inhabiting a particular part of the area. It also depends on a variety of environmental factors like sunlight, temperature; availability of nutrients and photosynthetic capacity of plants. Therefore it differs with different types of ecosystems.