Final answer:
Competition for food leading to malnutrition is a density-dependent limiting factor because it worsens with increased fish population density, resulting in higher mortality due to insufficient resources.
Step-by-step explanation:
The correct answer to this question is A) Competition for food leading to malnutrition. This is an example of a density-dependent limiting factor because it directly correlates with the population density of the fish. As the population of small fish in a lake increases, the competition for resources such as food also increases. If there is not enough food available to accommodate the growing population, some fish may not consume enough nutrients to survive, leading to malnutrition and even death. This reduction in population due to a lack of ample resources intensifies when fish are living in higher densities, therefore confirming that competition for food is a density-dependent factor.