The correct answer is diffusion.
Aquatic respiration refers to the procedure whereby an aquatic animal attains oxygen from water. Fish and other aquatic species utilize gills to consume oxygen dissolved in the water and diffuse carbon dioxide out of the bloodstream.
Diffusion is a procedure in which a substance moves from a region of high concentration to the region of low concentration until equilibrium is attained. In the case of aquatic species, blood with a low concentration of oxygen molecules moves through the gills. The concentration of the oxygen molecules in water is greater in comparison to the concentration of oxygen molecules in the gills.
As a consequence, the molecules of oxygen diffuse from water (higher concentration) to a region of lower concentration, that is blood. In the same manner, the molecules of carbon dioxide diffuse from the blood, that is, high concentration to water, that is, low concentration.
