Which of the following is a macronutrient?

Answer: Option C

Magnesium is the macronutrient among the given options.
Magnesium (Mg) is one of three secondary macronutrients, along with calcium and sulphur, required for balanced crop nutrition.
Mg²⁺ plays a role in several physiological processes that support plant growth and development.
Mg deficiencies can lead to reduced crop growth and yield. Magnesium deficiency is common in tomatoes, apples, grape vines, raspberries, roses and rhododendrons.