Monomer units of Dacron polymer are:

We have to recall that Dacron is the common trade name for the synthetic polyester whose IUPAC name is polyethylene terephthalate, usually abbreviated as PET. Being a polyester, it is produced by a condensation polymerisation reaction between a di-acid and a di-alcohol (di-ol).

First we write the structural formulae of the two possible di-alcohols mentioned in the options:

$$$\text{ethylene glycol} = HO{-}CH_2{-}CH_2{-}OH$$$

$$$\text{glycerol} = HO{-}CH_2{-}CH(OH){-}CH_2{-}OH$$$

Next we write the two possible di-acids (dibasic acids) that appear in the options:

$$$\text{phthalic acid} = HOOC{-}C_6H_4{-}COOH \quad\text{(benzene-1,2-dicarboxylic acid)}$$$

$$$\text{terephthalic acid} = HOOC{-}C_6H_4{-}COOH \quad\text{(benzene-1,4-dicarboxylic acid)}$$$

In the manufacturing of Dacron (PET) the di-acid used is specifically the para-isomer, i.e. terephthalic acid, because its -COOH groups lie opposite each other on the benzene ring, giving a linear repeating unit that leads to strong, fibre-forming chains. Likewise, the di-alcohol employed is ethylene glycol, whose two -OH groups are separated by two carbon atoms, again favouring linearity.

Now we write the condensation polymerisation reaction that actually forms Dacron. A general step is

$$$n\,HO{-}CH_2{-}CH_2{-}OH\;+\;n\,HOOC{-}C_6H_4{-}COOH \;\longrightarrow\; \bigl[-O{-}CH_2{-}CH_2{-}O{-}CO{-}C_6H_4{-}CO-\bigr]_n \;+\;2n\,H_2O$$$

Each ester linkage $$-COO-$$ is produced by eliminating one molecule of water $$H_2O$$ from the reacting -OH of ethylene glycol and -COOH of terephthalic acid, and this process repeats $$n$$ times to build the long polymer chain whose repeating unit is $$-O{-}CH_2{-}CH_2{-}O{-}CO{-}C_6H_4{-}CO-$$.

Because the pair ethylene glycol $$HO{-}CH_2{-}CH_2{-}OH$$ and terephthalic acid $$HOOC{-}C_6H_4{-}COOH$$ uniquely gives this polymer, while the other combinations listed in the options either form branched polyesters or completely different materials, we conclude that these two are the required monomers of Dacron.

Hence, the correct answer is Option 3.