Final answer:
The monomer relevant to the formation of the polyester PET is ethylene glycol (HOCH₂CH₂OH), which condenses with terephthalic acid to create the polymer.
Step-by-step explanation:
The monomer that is one of the two monomers for the polyester mentioned is ethylene glycol (HOCH₂CH₂OH). Polyesters are formed through a condensation reaction between a diacid and a glycol. The diacid in this case is terephthalic acid, while the glycol is ethylene glycol. When they react, they produce Polyethylene terephthalate (PET), which is widely used in drink bottles and polyester fabrics. Ethylene glycol is also known as ethanediol as per some literature references.