Mg++ and Ca++ are excreted in fishes through:

Explanation

Option A is correct.The kidneys in fishes play a vital role in osmoregulation, helping maintain the balance of water and salts in their bodies despite different aquatic environments. In addition to filtering nitrogenous wastes, they are especially important in the excretion of divalent ions such as magnesium (Mg²⁺) and calcium (Ca²⁺), which, if accumulated, can disturb cellular and physiological processes. Marine fishes actively excrete these excess salts through their kidneys and gills, while freshwater fishes conserve essential ions and excrete large amounts of dilute urine to maintain osmotic balance. Thus, fish kidneys are central to both waste removal and the regulation of ionic composition of body fluids.