The main cause of basicity of oxoacids of phosphorus is:

The correct answer is Option (2) → \(P-OH\) bonds

In the oxoacids of phosphorus, only the hydrogen atoms attached to oxygen in P–OH groups are ionisable and give \( \ce{H^{+}} \), so the basicity equals the number of P–OH bonds. For example, \( \ce{H3PO3} \) has two P–OH bonds and one P–H bond, so it is dibasic; \( \ce{H3PO4} \) has three P–OH bonds and is tribasic; \( \ce{H3PO2} \) has one P–OH bond and is monobasic.

Why the other options are incorrect

• Option (1) P–H bonds — Incorrect. P–H hydrogens are not ionisable and play no role in basicity; they give the acids their reducing character.

• Option (3) H₃–PO₃ bonds — Incorrect. There is no such bond; \( \ce{H3PO3} \) is the formula of phosphorous acid, not a type of bond.

• Option (4) Hydrogen bonds — Incorrect. Hydrogen bonding affects physical properties such as viscosity and boiling point, but it does not decide how many protons the acid can give.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Oxoacids of Phosphorus – “… The acids which contain P–H bond have strong reducing properties. Thus, hypophosphorous acid is a good reducing agent as it contains two P–H bonds and reduces, for example, AgNO₃ to metallic silver.
4AgNO₃ + 2H₂O + H₃PO₂ → 4Ag + 4HNO₃ + H₃PO₄
These P–H bonds are not ionisable to give H⁺ and do not play any role in basicity. Only those H atoms which are attached with oxygen in P–OH form are ionisable and cause the basicity. Thus, H₃PO₃ and H₃PO₄ are dibasic and tribasic, respectively as the structure of H₃PO₃ has two P–OH bonds and H₃PO₄ three.”