Concept:A monomer is the small repeating unit that joins together to form a polymer.Explanation:Polythene, also called polyethylene, is made by polymerising many units of ethene.Ethene has the formula C2H4 and contains a double bond between its carbon atoms.This double bond opens up during polymerisation, allowing ethene molecules to link in a chain.Therefore, ethene is the monomer of polythene.Answer:C. Ethene