Final answer:
Agricultural fertilizer runoff is the anthropogenic cause of eutrophication, leading to excessive algae growth and oxygen depletion, which creates dead zones in water bodies.
Step-by-step explanation:
The anthropogenic cause of eutrophication of a surface water body is d) Agricultural fertilizer runoff. Eutrophication occurs when runoff containing high levels of nutrients, especially nitrogen and phosphorus from human sources like agriculture and urban runoff, enters a body of water. This results in the overgrowth of algae, which upon dying, decompose and deplete the body of water of dissolved oxygen, creating hypoxic or 'dead zones' where most aquatic life cannot survive. The notion that fertilizer runoff decreases carbon dioxide concentration or increases oxygen concentration leading to dead zones, is incorrect. In fact, eutrophication due to fertilizer runoff significantly reduces oxygen levels in water, affecting the ecosystem's fauna adversely.