1. Key idea: In oxoacids of phosphorus, only hydrogens directly bonded to phosphorus are P–H bonds. Hydrogens present as –OH are O–H bonds, not P–H bonds.

2. Let us write the structures and count P–H bonds.

(i) $H_3PO_2$ (hypophosphorous acid)

Its structure is: $\mathrm{HOP(=O)H_2}$ So it contains:

• one $\mathrm{O-H}$ bond
• two $\mathrm{P-H}$ bonds

Hence, $H_3PO_2$ has 2 P–H bonds.

(ii) $H_3PO_3$ (phosphorous acid)

Its structure is: $\mathrm{(HO)_2P(=O)H}$ So it contains:

• two $\mathrm{O-H}$ bonds
• one $\mathrm{P-H}$ bond

Hence, $H_3PO_3$ has 1 P–H bond.

(iii) $H_4P_2O_5$ (pyrophosphorous acid)

This is formed by condensation of two molecules of $H_3PO_3$ with loss of one water molecule: $2H_3PO_3 - H_2O \rightarrow H_4P_2O_5$ Its structure is: $\mathrm{(HO)P(=O)-O-P(=O)(OH)H}$ Each phosphorus retains one $\mathrm{P-H}$ bond. So total number of P–H bonds = $1+1=2$

Hence, $H_4P_2O_5$ has 2 P–H bonds.

(iv) $H_4P_2O_6$ (hypophosphoric acid)

Its structure contains a $\mathrm{P-P}$ bond, and each phosphorus is bonded to two $\mathrm{OH}$ groups and one $=O$: $\mathrm{(HO)_2P(=O)-P(=O)(OH)_2}$ There is no P–H bond in this acid.

Hence, $H_4P_2O_6$ has 0 P–H bonds.

3. Now evaluate the options:

• A: $H_3PO_2$ and $H_4P_2O_5$
  Both have 2 P–H bonds each. ✅

• B: $H_4P_2O_5$ and $H_4P_2O_6$
  Second one has 0 P–H bonds. ❌

• C: $H_4P_2O_5$ and $H_3PO_3$
  $H_3PO_3$ has 1 P–H bond. ❌

• D: $H_3PO_3$ and $H_3PO_2$
  $H_3PO_3$ has 1 P–H bond. ❌

4. Therefore, the correct answer is: $\boxed{A}$