Which dissolved substance do aquatic animals remove from their external environment for use in cellular respiration

The correct answer would be oxygen.

Cellular respiration is the process of producing energy in the form of ATP by oxidation of the food (particularly carbohydrate or glucose).

Oxygen is one of the most important component required for complete oxidation of the glucose.

It is the final acceptor of the electron in the electron transport chain. It gets reduced to water.

Glucose + oxygen → carbon dioxide + water + ATP (approx 38).

Animals living on land gets their oxygen from the air they breathe in and animals living in the water extract dissolved oxygen from the surrounding water.

Fishes have specialized structures called gills which help them in extracting maximum amount of dissolved oxygen from the water.