Answer the questions on the basis of passage given below:
As aldehydes & ketones possess the carbonyl functional group, they show similar reactions. They exhibit nucleophilic addition reactions, reduction, oxidations, reactions due to ∝-Hydrogen atom, and many other reactions, like Cannizzaro reaction to electrophilic substitution reactions (in case of aromatic aldehydes & ketones). These reactions help in the preparation of a number of important compounds, like acetic acid, vinyl acetate, polymers and in perfume and dye industries. Nucleophilic addition reactions form cyanohydrins. acetals, alcohols and N-substituted derivatives like imine, oxime, semicarbazone, phenylhydrazone & Schiff's base. Reactions due to ∝-hydrogen atoms help in formation of aldols and a, B-unsaturated aldehydes & ketones.
In the following reaction
'A' is:
Answer & explanation
Correct answer: option 4
The correct answer is option 4. 
The reaction will be as follows:
The mechanism of this reaction can be easily described by two steps:
