1. Identify the first reaction: Reimer–Tiemann reaction

$p$-Cresol is $4$-methylphenol. When phenols react with chloroform in alkaline medium, they undergo the Reimer–Tiemann reaction, introducing a formyl group ($-CHO$) mainly at the ortho position to $-OH$.

So, $p$-cresol gives:

$A = 2\text{-hydroxy-}5\text{-methylbenzaldehyde}$

(One ortho position is equivalent to the other due to symmetry in $p$-cresol.)

2. Addition of HCN to aldehyde

Aldehyde $A$ reacts with HCN to form a cyanohydrin:

$Ar-CHO \xrightarrow{HCN} Ar-CH(OH)CN$

Thus,

$B = 2\text{-hydroxy-}5\text{-methylphenyl-}CH(OH)CN$

The carbon bearing $OH$ and $CN$ is attached to four different groups:

• $H$
• $OH$
• $CN$
• aryl group

So this carbon is stereogenic.

3. Acidic hydrolysis of cyanohydrin

On acidic hydrolysis, the nitrile group $-CN$ converts into carboxylic acid $-COOH$:

$Ar-CH(OH)CN \xrightarrow{H^+/H_2O} Ar-CH(OH)COOH$

Therefore the final product is:

$2\text{-hydroxy-}5\text{-methylphenyl-}CH(OH)COOH$

This is an $\alpha$-hydroxy acid.

4. Why is it chiral?

The carbon containing $OH$ is attached to:

• $H$
• $OH$
• $COOH$
• substituted aryl group

All four groups are different, so the acid is chiral.

5. Required structure

Hence the carboxylic acid must be:

$\boxed{\text{HO–}C_6H_3(CH_3)-CH(OH)COOH}$

with substituents corresponding to the product from ortho-formylation of $p$-cresol, i.e. 2-hydroxy-5-methylmandelic acid.

So the correct option is C.

6. Comparison with stored answer

Stored correct answer: C

Derived answer: C

They agree.