A

Acid etching creates microporosities in enamel that enhance micromechanical interlocking of resin, producing a tighter marginal seal and thus markedly reduces microleakage [Ortiz et al., 1979, PMID 364039; Retief et al., 1982, PMID 7045338]. The procedure does not affect polymerization shrinkage, the material’s thermal expansion coefficient, or internal porosity, making option A the primary and most advantageous indication.