Standing crop is defined as:

The correct answer is Option (1) → The mass of living material at a trophic level at a particular time

Option (1) The mass of living material at a trophic level at a particular time — Correct. The standing crop is the living material present at one trophic level at one moment, such as all the plants of a field today. It is measured as biomass or as the number of organisms in a unit area.

Option (2) Biomass per kilogram — Incorrect. Biomass is itself a mass, so “biomass per kilogram” has no meaning. The standing crop is expressed per unit area, and it belongs to a trophic level at a particular time.

Option (3) Crop of any plant in the field — Incorrect. This is the everyday farming meaning of “crop”. In ecology every trophic level has a standing crop, the herbivores and carnivores included, not only plants.

Option (4) The mass of dead and living organisms at a trophic level at a particular time — Incorrect. Dead organic matter is detritus, which is food for the decomposers. The standing crop counts only the living material.

NCERT Extract:
Energy Flow – …