Which among the following is classified as a condensation polymer?

First, we recall the basic definition of the two main polymerisation mechanisms studied in JEE chemistry.

A condensation polymer is formed when monomer molecules join together with the simultaneous elimination of small molecules such as $$H_2O,$$ $$HCl,$$ or $$CH_3OH.$$ The general step can be written as $$\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + \text{Small\;molecule.}$$

By contrast, an addition polymer is produced when unsaturated monomer units (usually containing a carbon-carbon double bond) link together without losing any small molecule. The simplified representation is $$n\,\text{Monomer} \rightarrow (\text{Monomer})_n.$$

Now we examine each given option in the light of these definitions.

We have Option A, Dacron. Dacron is the commercial name for polyethylene terephthalate. The reaction that produces it uses ethylene glycol $$HO-CH_2-CH_2-OH$$ and terephthalic acid $$HOOC-C_6H_4-COOH.$$ During polymerisation each hydroxyl group of ethylene glycol reacts with a carboxyl group of terephthalic acid, and every such step liberates a molecule of water. Symbolically,

$$\begin{aligned} n \, HO{-}CH_2{-}CH_2{-}OH &+ n \, HOOC{-}C_6H_4{-}COOH \\ &\rightarrow [-O{-}CH_2{-}CH_2{-}OOC{-}C_6H_4{-}CO-]_n + 2n\,H_2O. \end{aligned}$$

Because water is eliminated at every condensation step, Dacron is unmistakably a condensation polymer.

Next we consider Option B, Neoprene. Neoprene is obtained by the free-radical addition polymerisation of chloroprene $$CH_2=C(Cl)-CH=CH_2.$$ No small molecule is lost, so Neoprene is an addition polymer, not a condensation polymer.

Option C, Teflon, is polytetrafluoroethylene. Its monomer is tetrafluoroethylene $$CF_2=CF_2.$$ The polymerisation proceeds by addition of these double-bonded units, again without the elimination of any small molecule. Therefore Teflon is also an addition polymer.

Option D, Acrylonitrile, refers to the polymer polyacrylonitrile obtained from the monomer $$CH_2=CH-CN.$$ This, like the previous two, undergoes simple addition of the double bond and is therefore another addition polymer.

Among all four choices, only Dacron satisfies the defining condition of condensation polymerisation, namely, formation accompanied by the release of a small molecule (water).

Hence, the correct answer is Option A.