To solve this problem, we need to identify the compounds A, B, and C based on the given reactions.$\newline$Let's analyze the reactions step-by-step:$\newline$1. Compound A on treatment with Na gives B:$\newline$• Sodium (Na) reacts with alcohols to form alkoxides.$\newline$• Therefore, A is likely an alcohol.$\newline$• If A is $C_2H_5OH$ (ethanol), it reacts with Na to form $C_2H_5ONa$ (sodium ethoxide) and $H_2. \newline$2. Compound A on treatment with $PCl_5$ gives C:$\newline$• $PCl_5$ reacts with alcohols to form alkyl chlorides.$\newline$• Therefore, if A is $C_2H_5OH,$ it reacts with $PCl_5$ to form $C_2H_5Cl$ (ethyl chloride).$\newline$3. B and C react together to give diethyl ether:$\newline$• Sodium ethoxide (B) reacts with ethyl chloride (C) to form diethyl ether.$\newline$• This is a typical Williamson ether synthesis reaction.$\newline$Based on the above analysis, we can identify:$\newline$• A is $C_2H_5OH$ (ethanol)$\newline$• B is $C_2H_5ONa$ (sodium ethoxide)$\newline$• C is $C_2H_5Cl$ (ethyl chloride)$\newline$Therefore, the correct order of A, B, and C is:$\newline$Option 4: $C_2H_5OH, C_2H_5ONa, C_2H_5Cl.$