Final answer:
Walter is exhibiting an acute effect (option a), which is characterized by a rapid onset of symptoms following high-level exposure to a contaminant such as a pesticide.
Step-by-step explanation:
Walter feels dizzy after walking by a cornfield recently sprayed with a pesticide. He is manifesting an acute effect of exposure to a pollutant. Acute effects occur rapidly after exposure to a large amount of a substance and are typically associated with short-term, very high levels of exposure. In contrast, chronic effects result from exposure to lower amounts of a substance over a long period of time. Situations of acute exposure are more common in developing countries, but in developed countries, regulations have helped to control such incidents, making long-term low-level exposure a more prevalent issue. By definition, since Walter experienced dizziness immediately after exposure, it indicates an acute effect rather than chronic, subacute, or subchronic.