The formation of which of the following polymers involves hydrolysis reaction?

First, let us recall what is meant by a hydrolysis reaction. In hydrolysis, a covalent bond in a molecule is broken by the addition of water, and the molecule splits into two parts, each taking a fragment of the water molecule. When such a step is a part of a polymer-forming sequence, we say that the polymer is produced through hydrolytic polymerisation.

Now let us analyse each option one by one.

Bakelite. Bakelite is obtained from phenol and formaldehyde through the reaction

$$\text{Phenol} + \text{Formaldehyde} \rightarrow \text{Bakelite} + H_2O$$

Here water is eliminated; no bond is cleaved by water. Hence the process is condensation, not hydrolysis.

Nylon-6,6. Nylon-6,6 is prepared by the condensation of hexamethylenediamine with adipic acid:

$$n\bigl(H_2N\!(CH_2)_6NH_2\bigr)+n\bigl(HOOC\,(CH_2)_4\,COOH\bigr)\rightarrow \bigl[-NH\,(CH_2)_6\,NHCO\,(CH_2)_4\,CO-\bigr]_n+2n\,H_2O$$

Again, water is produced, not consumed; therefore the step is condensation, not hydrolysis.

Terylene (PET). Terylene is formed either from ethylene glycol and terephthalic acid or from ethylene glycol and dimethyl terephthalate. In both routes the key step is an esterification, producing water (or methanol) as a by-product:

$$n\,HOCH_2CH_2OH+n\,HOOC\text{-}C_6H_4\text{-}COOH\rightarrow \bigl[-OCH_2CH_2OCOC_6H_4CO-\bigr]_n+n\,H_2O$$

The mechanism is again condensation, not hydrolysis.

Nylon-6. Nylon-6 is produced from $$\varepsilon$$-caprolactam, whose formula is

$$(CH_2)_5C(O)NH$$

This is a cyclic amide (lactam) of six members. The polymerisation starts with the hydrolytic ring opening of caprolactam. We introduce water which breaks the amide bond:

$$\text{Caprolactam}+H_2O \rightarrow H_2N\,(CH_2)_5\,COOH$$

The product is $$\varepsilon$$-aminocaproic acid. As the reaction mixture is heated, the amino group and the carboxyl group of two such molecules condense, giving an amide linkage and releasing water; the sequence then repeats. Symbolically, after the initial hydrolysis step we have

$$n\,H_2N\,(CH_2)_5\,COOH\;\rightarrow\; \bigl[-NH\,(CH_2)_5\,CO-\bigr]_n + n\,H_2O$$

Thus the very first, indispensable step that initiates the polymer chain is the hydrolysis of the lactam ring. Therefore the overall polymerisation is commonly called hydrolytic ring-opening polymerisation.

Among the four given polymers, only the formation of Nylon-6 necessarily involves a hydrolysis reaction. The other three are formed solely by condensation without any bond cleavage by water.

Hence, the correct answer is Option D.