The three primary soil micro-nutrients are

The three primary soil micro-nutrients are

carbon, oxygen and water
copper, cadmium and carbon
nitrogen, phosphorus and potassium
boron, zinc and magnesium
This question was previously asked in
UPSC CAPF – 2013
The correct option is C. Nitrogen, Phosphorus, and Potassium are the three primary macronutrients required by plants.
Plants require various nutrients from the soil for healthy growth. These are broadly classified into macronutrients (needed in larger quantities) and micronutrients (needed in smaller quantities). The three most crucial macronutrients, often called primary nutrients, are Nitrogen (N), Phosphorus (P), and Potassium (K). While the question asks for “micro-nutrients”, option C lists the standard “primary” nutrients. Given the options, NPK is the only plausible set of primary nutrients listed.
Other macronutrients include Calcium (Ca), Magnesium (Mg), and Sulfur (S). Micronutrients include Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Boron (B), Molybdenum (Mo), and Chlorine (Cl). Option D lists some micronutrients, but NPK are universally referred to as the primary nutrients. The question’s wording “primary soil micro-nutrients” might be slightly imprecise, potentially intending to ask for primary nutrients or listing common soil nutrients from which to choose the primary ones. In the context of soil fertility and fertilizers, NPK are the standard primary nutrients.