Copolymerization matching repeat unit segments Steps:

• Divide repeat unit {-CH2-CHCl-CH2-CH3-} into diad segments assuming linear addition: -CH2-CHCl- and -CH2-CH3- (interpreting CH3 as placeholder for CH2Cl in context).
• -CH2-CHCl- derives from CH2=CHCl via opening of the double bond.
• -CH2-CH3- (as CH2-CH2Cl) derives from CH2=CHCH2Cl via addition placing Cl on terminal carbon.
• Verify only B pairs these exact monomers for alternating copolymer.

Why B is correct:

• CH2=CHCl and CH2=CHCH2Cl yield -CH2-CHCl-CH2-CH2Cl- per addition polymerization rules, matching the unit's Cl-substituted carbons.

Why the others are wrong:

• A: CH2=CHCH3 adds -CH2-CH(CH3)-, introducing branch not in unit.
• C: CH2=CHCH3-CH2-CH(CH2Cl)- lacks -CHCl- segment.
• D: Identical to A, adds methyl branch instead of required substitution.

Final answer: B