Option C. Soil Reaction is the correct answer.
Among the properties of soil, Soil Reaction or Soil pH, has the greatest effect on plant and micro-organism growth. It is actually an indication of acidity or alkalinity of soil. It is measured in pH units (0-14). Soil Reaction influences the availability of many nutrients--Nitrogen, Phosphorous, calcium, Magnesium, Potassium, Iron, Manganese, Zinc etc.