Fluid balance and regulation within the body is achieved through an intricate process involving various signals, messengers and hormones. Several minerals are vital to this process but some are more important than the others.
Divalent minerals like Calcium (C) and Magnesium (D) are involved primarily in muscle and bone physiology. Calcium also plays a role in cell signaling.
Chloride (A) and Sodium (B) are more important in regulating fluid in the body, particularly, the latter cation is more important. (Since only one option is correct, as per the grammar of the question framed, we must choose sodium!)
Therefore, option (B) is correct.