Final answer:
A trophic cascade occurs when predators limit the density and/or behavior of their prey, which enhances the survival of the next lower trophic level, often resulting in significant alterations to ecosystem structure.
Step-by-step explanation:
A trophic cascade is an ecological phenomenon triggered by the addition or removal of top predators and involving reciprocal changes in the relative populations of predator and prey through a food chain, which often results in dramatic changes in ecosystem structure and nutrient cycling. In a trophic cascade, when predators limit the density and/or behavior of their prey, this not only affects the prey species but also enhances the survival of the next lower trophic level. This typically occurs because the reduced prey population releases the pressure on the species at the next lower trophic level that the prey would normally consume or suppress.
The correct answer to the question is:
d. when predators limit the density and/or behavior of their prey and thereby enhance survival of the next lower trophic level.