The compound A with following sequence of reaction gave benzoic acid

A. Nitrobenzene
B. Aniline
C. Benzaldehyde
D. Amides

Hint: When aniline undergoes a reaction with nitrous acid at the temperature of 273K -278K results in the formation of benzenediazonium chloride. The production of nitrous acid is due to the reaction of sodium nitrite with hydrochloric acid.

Complete Step by Step Solution:
In the given question, the reactant A undergoes reaction with \[{\rm{NaN}}{{\rm{O}}_{\rm{2}}}/{\rm{HCl}}\] to form B. The then product formed B reacts with KCN to form C. The acidic hydrolysis of C results in the formation of benzoic acid.

Let’s understand the diazotization reaction in detail. In the diazotization reaction, the primary aromatic amine converts into diazonium salt. Here, A undergoes undergoes reaction with \[{\rm{NaN}}{{\rm{O}}_{\rm{2}}}/{\rm{HCl}}\]. So, the reaction is,

Image: Reaction of aniline with \[{\rm{NaN}}{{\rm{O}}_{\rm{2}}}/{\rm{HCl}}\]

In the second reaction, B undergoes a reaction with KCN. The reaction of diazonium chloride with KCN results in the formation of cyanide. So, the reaction is,

Image: Reaction of diazonium chloride with KCN
So, product C is benzonitrile.
Let’s understand the third reaction. The acidic hydrolysis of benzonitrile gives benzoic acid.

Image: Acidic hydrolysis of benzonitrile

So, the complete reaction is,

Image: The reaction of aniline to form benzoic acid
Hence, the reactant A is aniline, i.e, \[{{\rm{C}}_{\rm{6}}}{{\rm{H}}_{\rm{5}}}{\rm{N}}{{\rm{H}}_{\rm{2}}}\] .

Note: The Sandmeyer reaction is the one in which chloride, bromide and cyanide ion can be introduced in the benzene ring in the presence of copper ion. And, we know, the acidic hydrolysis of cyanide gives the carboxylic acid.