C

During polymerization of methyl-methacrylate (density ≈0.94 g/cm³) to polymethyl-methacrylate (density ≈1.19 g/cm³), mass is conserved, so the rise in density corresponds to a volume contraction of about 21 %—a value experimentally confirmed for methacrylate esters (change in molar volume ≈22 cm³/mol) [Patel et al., 1987, PMID 3828447].