Net Primary Productivity refers to:

Net Primary Productivity (NPP) is defined as the amount of energy that remains after plants have utilized a portion of the energy they capture through photosynthesis for their own metabolic processes, primarily respiration. This means that NPP represents the energy that is available for growth and reproduction in plants, and, by extension, it serves as a foundational energy source for the entire food web, supporting primary consumers and higher trophic levels.

In contrast, total energy produced in a habitat refers to Gross Primary Productivity (GPP), which does not account for the energy consumed by plants in respiration. The biomass of primary consumers represents the total mass of organisms that eat plants, which is separate from the calculation of productivity. The efficiency of photosynthesis pertains to how effectively plants convert sunlight into chemical energy, but does not directly define NPP itself. Thus, the correct answer accurately captures the concept of NPP by highlighting the available energy after accounting for plant respiration.