Final answer:
a. Freshwater fish use ATP to pump ions inward across the gill membrane. Freshwater fish use ATP to pump ions inward across the gill membrane, gills extract oxygen from the water, the larger the fish, the larger the gill surface area must be to support its metabolism.
Step-by-step explanation:
The best statement regarding fish gills that approximates the description of a mechanism is option a. Freshwater fish use ATP to pump ions inward across the gill membrane. In fish, gills have evolved as an organ to extract oxygen from the water. Gills consist of thin, folded tissues that provide a large surface area for oxygen uptake. The statement in option d, which states that the larger the fish, the larger the gill surface area must be to support its metabolism, is also true.
This statement describes a mechanism involving the active transport of ions across the gill membrane by freshwater fish. In freshwater environments, fish need to actively pump ions inward to maintain osmotic balance and prevent the influx of water into their bodies. This process requires energy in the form of ATP, making it a mechanistic explanation of a physiological process in fish gills.