A

Reasoning: In the maxillary first molar the two mesial pulp horns project highest toward the occlusal surface; they lie significantly closer to the enamel than the distal horns (≈0.80 mm vs 0.41 mm above the CEJ) [Gaba et al., 2019, PMID 31367104]. Micro-CT studies likewise show the mesiobuccal horn is the closest pulp extension to the outer surface of the crown [Baltacıoğlu et al., 2018, PMID 30775706]. Because a Class II preparation removes mesial (or distal) proximal and occlusal tooth structure, the elevated, shallow mesial horns—the mesiobuccal and mesiolingual—are the ones most likely to be inadvertently exposed.