Phosphorus forms a number of oxoacids which differ in their structures and oxidation state of phosphorus. All the acids contain phosphorus atom//atoms linked tetrahedrally to four other atoms or groups. Each of them has at least one `P=O` or `P rarr O` unit and one `P-OH` unit. The `OH` group is ionisable but `H` atom linked directly to `P` is non-ionisabl. Structures of all the acids are considered to be derived either from phosphorus acid or phosphoric acid.
Which one is monobasic acid ?
A. `H_3 PO_2`
B. `H_3 PO_3`
C. `H_3 PO_4`
D. `H_3 PO_5`
