The option that would have a large impact on the environment is:
a population of robins die.
The death of a population can significantly impact the ecosystem, affecting food chains, nutrient cycling, and other species that may rely on robins for food or other ecological roles. In contrast, the death of a single robin or the abandonment of a nest has a much smaller impact. Competing for food may indicate a struggle but does not directly affect population size or ecosystem balance in the same way.