Look at the structure of lactic acid:
Lactic acid is a monomer that reacts to form polylactic acid (PLA).
Lactic acid contains two different functional groups that allow it to undergo polymerisation.
What are these two functional groups?
−OH-\text{OH}−OH and −C=O-\text{C}=\text{O}−C=O
−OH-\text{OH}−OH and −CH3-\text{CH}_3−CH3
−OH-\text{OH}−OH and −COOH-\text{COOH}−COOH
−CH3-\text{CH}_3−CH3 and −COOH-\text{COOH}−COOH