Consider the following sequence of reactions

The product 'B' is

Step 1: Aniline reacts with NaNO₂ at 0-5°C (diazotization):

$$C_6H_5NH_2 + NaNO_2 + HCl \xrightarrow{0-5°C} C_6H_5N_2^+Cl^-$$

Product 'A' is benzenediazonium chloride.

Step 2: Benzenediazonium salt couples with N,N-dimethylaniline (azo coupling):

The diazonium salt attacks the para position of N,N-dimethylaniline (which is a strongly activated ring).

Product 'B' is an azo compound: p-(N,N-dimethylamino)azobenzene (also known as butter yellow or methyl yellow).

Structure: C₆H₅-N=N-C₆H₄-N(CH₃)₂ (with the -N=N- azo bridge connecting the two aromatic rings, NMe₂ at para position).

This matches option 3.