D

Reasoning: The reciprocal arm must be rigid so it can counteract the lateral force generated when the flexible retentive arm passes over the height of contour. To preserve this rigidity its cross-section is kept essentially uniform along the length, i.e., it is not tapered, unlike the retentive arm that is tapered for flexibility [Eliason, 1983, PMID 6337247; Camacho et al., 2018, PMID 29687083].