The number of functional plant nutrient is

Answer: Option A

The number of functional plant nutrients is 20.

Plants require various nutrients for their growth and development. The essential nutrients for plants can be broadly categorized into macronutrients and micronutrients. Macronutrients are required by plants in relatively large quantities, while micronutrients are needed in smaller amounts.

The macronutrients essential for plant growth include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). Additionally, plants require micronutrients such as iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl), and nickel (Ni), among others.

In total, there are 20 functional plant nutrients that are essential for plant growth and development.

Therefore, Option B: 20 is the correct number of functional plant nutrients.