Which of the following is a source of potassium fertilizer?

Potassium chloride (KCl) is a well-known and widely used source of potassium fertilizer. Potassium is an essential nutrient for plant growth, and KCl is favored because it has a high potassium content, typically around 60% soluble potash (K2O). This makes it highly effective for delivering potassium to plants, enhancing their ability to withstand stress, promote root development, and improve overall yield.

The other options do not provide potassium in a suitable form for fertilization. Sodium chloride (NaCl) is common table salt and primarily contributes sodium and chlorine, neither of which fulfill the potassium requirement for plants. Ammonium nitrate (NH4NO3) is a nitrogen fertilizer and does not contain potassium. Calcium carbonate (CaCO3) is primarily a source of calcium and carbon, used mainly to raise soil pH or supply calcium but is not associated with potassium fertilization. Thus, among the choices provided, KCl is the clear and effective source of potassium fertilizer.