Matching repeat unit to monomer and polymerization type

Steps:

• Examine the diagram's repeat unit: -[CH2-CH2]-.
• Recognize this as the structure from ethylene monomer.
• Confirm addition polymerization: double bond opens without losing atoms.
• Rule out alternatives by structure mismatch or wrong mechanism.

Why A is correct:

• The repeat unit -[CH2-CH2]- matches addition polymerization of H2C=CH2, where the alkene adds across the double bond per the addition reaction definition.

Why the others are wrong:

• B: Condensation polymerization eliminates small molecules like water; this polymer shows no such loss.
• C: H3C-CH=CH2 (propene) yields -[CH2-CH(CH3)]- repeat unit, not -[CH2-CH2]-.
• D: H3C-CH3 (ethane) lacks a reactive double bond for polymerization; condensation requires functional groups like -OH or -COOH.

Final answer: A