Correct Answer – 3
The oxoacids which contain `P-H` bond have strong reducing properties. Thus ,hypophosporous acid is a good reducing agent as it contains two `P-H` bonds. For example ,it reduces silver nitrate to metallic silver.
`4AgNO_(3)+2H_(2)O+H_(3)PO_(2)rarr4Ag+4HNO_(3)+H_(3)PO_(4)`
The `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` bonds are ionisable and cause bacity .Thus ,hyprophosphorous is monobasic as its structure has only one `P_OH` bond. On the other hand, `H_(3)PO_(3)` and `H_(3)PO_(4)` are dibasic and tribasic,respectively as the structure of `H_(3)PO_(3)` has two `P_OH` bonds and `H_(3)PO_(3)` three.

Correct Answer – D
`H_(3) PO_(2)` is monobasic and is reducing due to `P – H` bond.