At pH 4.0 the predominant ionic form of phosphorus present is

Answer: Option B

Phosphorus is most abundant in the soil at pH 6 to 7, but at pH 4, i.e. in acidic soils, the dominant ionic form of phosphorus present is orthophosphates, i.e., $${\text{H}}_2{\text{PO}}_4^{-}.$$   Availability of phosphorus decreased with an increase in pH. But to maintain the availability of phosphorus at a high pH, the percentage of organic matter in the soil is important.