The pair that contains two $$P - H$$ bonds in each of the oxoacids is:

First we recall a very useful thumb-rule for oxo-acids of phosphorus: only those hydrogen atoms that are attached directly to the phosphorus atom constitute $$P-H$$ bonds; any hydrogen written as part of an $$OH$$ group is bonded to oxygen and does not give a $$P-H$$ bond.

With this idea fixed, we examine every acid mentioned in the options one after another, writing its accepted structural formula and then counting the $$P-H$$ links.

We begin with hypophosphorous acid $$H_3PO_2$$. A standard way of writing its structure is

$$H-P(=O)(OH)H$$

Here two hydrogens are attached directly to phosphorus and one hydrogen is on an oxygen. Therefore

$$H_3PO_2:\; \text{number of }P-H\text{ bonds}=2$$

Next comes phosphorous acid $$H_3PO_3$$. Its structure is

$$H-P(=O)(OH)_2$$

Now there is only one hydrogen directly on phosphorus; the other two hydrogens belong to $$OH$$ groups. Hence

$$H_3PO_3:\; \text{number of }P-H\text{ bonds}=1$$

We turn to hypodiphosphorous (pyrophosphorous) acid $$H_4P_2O_5$$. It is obtained by condensation of two $$H_3PO_2$$ units with loss of one water molecule, and its accepted structure is

$$HO-P(H)-P(H)-OH$$

Each phosphorus atom carries one hydrogen directly, so altogether there are two $$P-H$$ bonds:

$$H_4P_2O_5:\; \text{number of }P-H\text{ bonds}=2$$

Finally we look at $$H_4P_2O_6$$ (hypophosphoric acid). Its structure contains a $$P-O-P$$ linkage with every hydrogen attached to oxygen, for example

$$(HO)_2P-O-P(OH)_2$$

Consequently, there is no hydrogen directly attached to phosphorus, giving

$$H_4P_2O_6:\; \text{number of }P-H\text{ bonds}=0$$

Now we collect the counts:

$$H_3PO_2 \rightarrow 2,\;\; H_3PO_3 \rightarrow 1,\;\; H_4P_2O_5 \rightarrow 2,\;\; H_4P_2O_6 \rightarrow 0$$

We need a pair in which each acid possesses exactly two $$P-H$$ bonds. From the list we see that only $$H_3PO_2$$ and $$H_4P_2O_5$$ satisfy this requirement simultaneously.

Hence, the correct answer is Option D.