B

An osseous “crater” is a two-wall interdental defect in which the facial and lingual (buccal and palatal) crestal plates are resorbed, forming a concavity, while the radicular (proximal root) bone surfaces remain intact—precisely the pattern described in the question [Wu et al., 2001, PMID 11565248; Vrotsos et al., 1999, PMID 9923510].