At pH 4.0, the predominant ionic form of phosphorus present is

✅ Explanation:
-The availability of different forms of phosphorus in soil is highly dependent on pH. At a pH of 4.0 (acidic), the predominant form of inorganic phosphorus is H₂PO₄⁻ (dihydrogen phosphate ion).

📌 Other Options Explanations:
-(a) HPO₄⁻²: This form (hydrogen phosphate ion) is more prevalent at slightly higher pH levels, around neutral to slightly alkaline conditions.
-(c) PO₄⁻³: This form (phosphate ion) is the dominant form in highly alkaline soils.