Final answer:
A cornfield is the least stable ecosystem compared to a rainforest, savanna, or coral reef due to its reliance on human intervention and greater susceptibility to soil erosion and biodiversity loss.
Step-by-step explanation:
Out of the ecosystems listed, the least stable is generally considered to be B. A cornfield. Cornfields are human-made agricultural ecosystems and are often subject to soil erosion, loss of biodiversity, and reliance on human intervention for maintenance. In contrast, rainforests, savannas, and coral reefs are natural ecosystems that have evolved over millennia with intricate and resilient ecological processes, though they can be sensitive to disturbances too.
When planning an ecological preserve, conservationists often look for areas with rich biodiversity, such as a tropical rain forest, due to its highly diversified niches that provide habitats for a wide diversity of species.
Regarding other ecosystems, the arctic tundra is characterized by short growing seasons, islands can be particularly vulnerable to exotic species, deserts have very little rainfall, and the benthic realm of the ocean hosts organisms like squids.