The correct answer is A. Calcium carbonate.
Alkalinity is a measure of the ability of water to neutralize acids. It is expressed in milligrams per liter (mg/L) as equivalent calcium carbonate (CaCO3). This means that 1 mg/L of CaCO3 is equivalent to 1 mg/L of any other substance that has the same ability to neutralize acids.
Calcium carbonate is the most common form of alkalinity in water. It is found in limestone, dolomite, and other minerals. Magnesium carbonate is also a common form of alkalinity, but it is not as prevalent as calcium carbonate. Sodium carbonate and calcium hydroxide are not as common forms of alkalinity, but they can be found in some waters.
Alkalinity is important because it helps to protect fish and other aquatic life from the harmful effects of acids. It also helps to prevent corrosion of pipes and other equipment.