Final answer:
Nutrient runoff from fertilizers and sewage causes eutrophication that leads to oxygen depletion and the death of aquatic life, resulting in ocean dead zones.
Step-by-step explanation:
Among the options provided, nutrient runoff can lead to the formation of an ocean dead zone. When excess nutrients, particularly nitrogen and phosphorus from fertilizer runoff and sewage, enter water bodies, they fuel the overgrowth of algae. This process, known as eutrophication, results in massive algal blooms that eventually die off. The decomposition of these dead algae by bacteria consumes dissolved oxygen in the water, making the environment hypoxic, or low in oxygen. This lack of oxygen can lead to the death of aquatic life, creating a dead zone.
Overfishing and rise in sea levels do not directly cause dead zones. Although increased oxygen levels would seem beneficial, in the context of dead zones, it is the depletion of oxygen that is relevant. Therefore, the correct answer is nutrient runoff, as it leads to a cycle of eutrophication and subsequent oxygen depletion, causing dead zones.