Match reaction (P):

$\mathrm{P_2O_3 + 3H_2O \rightarrow 2H_3PO_3}$

Here $\mathrm{P_2O_3}$ is the anhydride of phosphorous acid.

So, $P \rightarrow (2)\; \mathrm{H_3PO_3}$

Match reaction (Q):

White phosphorus reacts with alkali and water to give phosphine:

$\mathrm{P_4 + 3NaOH + 3H_2O \rightarrow PH_3 + 3NaH_2PO_2}$

Since the question asks to match with one of the products, we identify $Q \rightarrow (3)\; \mathrm{PH_3}$

Match reaction (R):

$\mathrm{PCl_5}$ reacts with carboxylic acids to form the corresponding acyl chloride:

$\mathrm{CH_3COOH + PCl_5 \rightarrow CH_3COCl + POCl_3 + HCl}$

Among the listed products, we have $R \rightarrow (4)\; \mathrm{POCl_3}$

Match reaction (S):

Hypophosphorous acid is a strong reducing agent. It reduces $\mathrm{Ag^+}$ to Ag and itself gets oxidized to phosphoric acid:

$\mathrm{H_3PO_2 + 2H_2O + 4AgNO_3 \rightarrow H_3PO_4 + 4Ag + 4HNO_3}$

Therefore, $S \rightarrow (5)\; \mathrm{H_3PO_4}$

Final matching:

$P \rightarrow 2,\quad Q \rightarrow 3,\quad R \rightarrow 4,\quad S \rightarrow 5$

Compare with options:

This corresponds to Option D.

Verification with stored answer:

Stored correct answer = D

Our derived answer = D

Hence, they agree.