The correct answer is option C, standing crop.

Explanation:
- The mass of living material at a trophic level at a particular time is referred to as the standing crop.
- The standing crop represents the total biomass of organisms present at a specific trophic level in an ecosystem at a given point in time.
- It includes all the living organisms, such as plants, animals, and microorganisms, at that trophic level.
- The standing crop is measured in terms of the dry weight of the organisms per unit area or volume.

Importance of standing crop:
- The standing crop is an important ecological parameter as it provides information about the amount of energy available in an ecosystem at a specific trophic level.
- It helps in understanding the productivity and functioning of the ecosystem.
- By measuring the standing crop, scientists can estimate the amount of energy available to support higher trophic levels and the overall health of the ecosystem.

Difference between standing crop and net primary productivity:
- Net primary productivity (NPP) is the rate at which energy is stored in the organic matter of plants through photosynthesis, minus the energy lost through respiration.
- NPP represents the energy available to the primary consumers in an ecosystem.
- On the other hand, standing crop refers to the total biomass of organisms at a specific trophic level, including plants, animals, and microorganisms.
- While NPP represents the rate of energy production, standing crop represents the total amount of energy stored at a given point in time.

In conclusion, the correct answer to the question is option C, standing crop, as it is the term used to describe the mass of living material at a trophic level at a particular time.