Hydrolysis of which compound will give carbolic acid?

We need to determine which compound yields carbolic acid upon hydrolysis. Carbolic acid is the common name for phenol ($$C_6H_5OH$$). To this end, each option is examined for its hydrolysis products.

In the case of Option A: Cumene (isopropylbenzene), simple hydrolysis does not occur. Instead, cumene undergoes oxidation by $$O_2$$ in the industrial cumene process to afford phenol and acetone, but this transformation is an oxidation rather than hydrolysis.

When Option B: Benzenediazonium chloride ($$C_6H_5N_2^+Cl^-$$) is warmed with water, hydrolysis proceeds to give phenol:

$$C_6H_5N_2^+Cl^- + H_2O \xrightarrow{\text{warm}} C_6H_5OH + N_2 + HCl$$

This reaction directly furnishes phenol (carbolic acid).

On hydrolysis, Option C: Benzal chloride ($$C_6H_5CHCl_2$$) yields benzaldehyde ($$C_6H_5CHO$$) rather than phenol:

$$C_6H_5CHCl_2 + H_2O \rightarrow C_6H_5CHO + 2HCl$$

Furthermore, hydrolysis of Option D: Ethylene glycol ketal regenerates the corresponding carbonyl compound along with ethylene glycol, without producing phenol.

Therefore, the correct choice is Option B: Benzenediazonium chloride.