In an ecosystem, the trophic levels represent different layers of the food chain, with each level consisting of organisms that share the same function in the food web. The first trophic level is made up of primary producers, such as plants and phytoplankton, which are abundant due to their ability to convert sunlight into energy through photosynthesis.
As we move up each trophic level, the number of organisms tends to decrease. This is primarily due to the energy loss that occurs at each step of the food chain. When a predator consumes its prey, only a portion of the energy is converted to biomass; much of it is lost as heat or used for metabolic processes. Consequently, the higher you go in the trophic levels, the fewer organisms can be supported.
Therefore, among the options provided, the fourth trophic level is most likely to contain the smallest population of organisms. This level often consists of top predators, which are fewer in number compared to the primary producers and herbivores that occupy the lower trophic levels.