Which of the following best describes an osmoregulator?

An osmoregulator is best described as an organism that maintains body fluids with a different concentration than that of the surrounding environment. This ability allows osmoregulatory organisms to actively control the osmotic pressure within their bodies, ensuring that their internal environment remains stable despite changes in external conditions. For example, many freshwater fish actively take up salts and excrete excess water to counteract the dilution of their body fluids by their water-rich environment.

This control is particularly important for the survival of these organisms, as imbalances in osmotic pressure can lead to cellular damage and affect overall physiology. Therefore, the defining characteristic of osmoregulators is their ability to maintain this difference in osmotic concentration, making option B the correct choice.