Monomer of Novolac is:

We begin by recalling what a Novolac resin is. Novolac is the product obtained when phenol reacts with formaldehyde ($$\text{HCHO}$$) in an acidic medium. The first step of this condensation reaction is the electrophilic substitution of formaldehyde at the ortho and para positions of the phenolic ring.

Using the general mechanism of electrophilic aromatic substitution, we can state:

$$\text{Phenol} + \text{HCHO} \;\xrightarrow[\text{acid}]{\text{condensation}}\; o\text{-Hydroxymethyl-phenol} \;+\; p\text{-Hydroxymethyl-phenol}$$

Here we clearly see that the immediate product formed—before the polymer chain starts growing—is the substituted phenol that carries a -CH2OH group at the ortho or para position with respect to the -OH already present on the ring. These substituted phenols then link through methylene (-CH2-) bridges to give the linear Novolac polymer. Therefore, each repeating unit (monomeric unit) of the polymer chain is precisely this $$o$$-hydroxymethylphenol (and its para analogue).

Among the options offered:

Option D states “o-Hydroxymethylphenol,” which matches exactly the compound produced in the first condensation step and which ultimately acts as the monomeric unit that repeats to form Novolac.

Hence, the correct answer is Option 4.