Standing crop is defined as:

The mass of living material at a trophic level at a particular time

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

Each trophic level within an ecosystem has a specific amount of living material present at a given point in time, which is referred to as the "standing crop." This standing crop can be quantified by measuring either the biomass (mass of living organisms) or the population number of organisms within a defined unit area.

Biomass is typically expressed in terms of either fresh weight (total mass of living organisms as they are) or dry weight (mass of living organisms after removing water content through drying). These measures of standing crop help ecologists and scientists study the energy flow and dynamics within ecosystems and understand the distribution of living organisms across different trophic levels.