The correct option is "a", because the given statement is true.
Actually there is a steroid hormone that is behind the sodium concentration in the blood that is primarily regulated by the kidneys, that hormone is known as Aldosterone, it works in such a way that when our sodium levels drops then our body signals to the adrenal glands so that these glands release aldosterone into our blood and after that the hormone aldosterone signals our kidneys to reabsorb sodium into the blood and that is how kidney regulates the sodium concentration.
