The best reasoning that explains the student's claim that there is less energy available in ecosystems without top-level predators is:
C. Lack of mountain lions reduces the number of trophic levels.
Having top-level predators like mountain lions helps maintain the balance of ecosystems by controlling the populations of herbivores and thereby allowing for a more diverse and stable community with multiple trophic levels. Without these predators, herbivore populations might explode, leading to overconsumption of vegetation, which can reduce the overall energy available in the ecosystem.