Identify product A in following reaction. $\mathrm{C}_6 \mathrm{H}_5 \stackrel{+}{\mathrm{N}_2} \mathrm{Cl}^{-} \xrightarrow{\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}} \mathrm{~A}+\mathrm{N}_2+\mathrm{CH}_3 \mathrm{CHO}+\mathrm{HCl}$

Benzenediazonium chloride with ethanol undergoes reduction where the diazonium group is replaced by hydrogen.

The diazonium salt $\mathrm{C_6H_5\stackrel{+}{N_2}Cl^{-}}$ is electrophilic and ethanol acts as a reducing agent, donating hydrogen while being oxidized to acetaldehyde. Nitrogen gas evolves, and hydrochloric acid forms from the chloride ion.

The balanced reaction is:

$\mathrm{C_6H_5\stackrel{+}{N_2}Cl^{-}} + \mathrm{C_2H_5OH} \rightarrow \mathrm{C_6H_6} + \mathrm{N_2} + \mathrm{CH_3CHO} + \mathrm{HCl}$

Comparing with the given transformation, A is benzene $\mathrm{(C_6H_6)}$.

Final answer: $\boxed{\text{A}}$