C

Deflexion (partial extension) of the fetal head prevents the smallest (sub-occipito-bregmatic) diameter from entering the pelvis, so the head often engages with the occiput directed posteriorly; intrapartum sonographic studies show that OP rotation is significantly more common when the head is deflexed than when it is well-flexed [Bellussi 2017, PMID 28399531; Bellussi 2020, PMID 33345926].