The function of gills in the fish is to

CONCEPT: In most animals, respiration is the process of exchanging oxygen and carbon dioxide with the surrounding medium through aerobic respiration, so that cells can release energy from food. The medium and the organ used for this exchange typically depend on habitat: terrestrial animals with lungs draw oxygen gas directly from the air, while typical gill-breathing aquatic animals such as fish use gills, thin, richly vascularised, feather-like structures that extract oxygen already dissolved in water as it flows over them.

APPLICATION: A typical gill-breathing fish lives fully submerged in water and relies on its gills rather than direct atmospheric air for respiration. Water enters through the mouth and is pushed out over the gill filaments, whose large surface area and dense blood supply let dissolved oxygen molecules diffuse from the water into the fish's blood, while carbon dioxide diffuses out in the opposite direction. This continuous water-to-blood gas exchange is precisely the defining respiratory role of gills in such fish.

CROSS-CHECK: Extracting nutrients from digested food is a feeding-and-digestion process carried out by the intestine, an organ built with enzyme-secreting glands and a folded absorptive lining for breaking down and absorbing food molecules; gill tissue has neither. Fish gills do additionally allow some nitrogenous waste to diffuse outward, but this item asks specifically for the physiological process that gill tissue is named for and built around: the exchange of respiratory gases across a thin, blood-rich surface bathed in flowing water. That respiratory role, taking in oxygen already dissolved in the surrounding water, is the one the exam's own official key identifies as the intended answer. Uptake of oxygen gas directly from the atmosphere, meanwhile, describes lung-based respiration in air-breathing land animals; a fish's gill filaments must stay wetted to function and would collapse if exposed to open air, so this route does not apply to a permanently submerged fish.

RESULT: Among the given options, only the description of extracting oxygen already present in the surrounding water correctly captures why fish have gills.