The relationship between the size of individual organisms and the number of organisms in a given trophic level is D) inversely proportional.
This is because a larger organism must consume more in order to sustain itself. Therefore, there can either be more organisms of smaller size or less organisms with larger size, because the energy entering a given trophic level is limited.
This is also the basis of formation of biomass pyramids, the total mass at higher trophic levels decreases.