Which of the foll A. H₃PO₂

B. H₃BO₃

C. H₃PO₄

D. H₃PO₃

H₃PO₂ is monobasic as it consist of only one OH group.

Structure of H₃PO₂:

H₃BO₃: it does not act as proton donor but behaves as lewis acid. Boric acid does not dissociate in aqueous solution. It can be predicted from its structure that it have 3 OH group. Boric acid first accept one OH^- then it liberates one electron per molecule. Therefore it is monobasic.

H₃PO₃: it is dibasic as it have two OH group.

structure of H₃PO₄:

It has three OH group therefore it is tri basic. Oxidation state of phosphorous in H₃PO₄ is +5.

As it has three OH group, it means that it has 3 ionisable hydrogen atoms and hence form three series of salts. These are:

1.NaH₂PO₄

2. NaHPO₄

3. Na₃PO₄

Hence the correct option is (iii).