Final answer:
The sacrotuberous ligament reinforces the joint that becomes mobile during late pregnancy and childbirth inferiorly, making the correct answer A) Sacrotuberous ligament.
Step-by-step explanation:
The joint that is only mobile during late pregnancy and childbirth is reinforced inferiorly by the sacrotuberous ligament. Answer to this question would be Option A) Sacrotuberous ligament. The sacrotuberous ligament spans from the sacrum to the ischial tuberosity. This ligament, along with the sacrospinous ligament, helps to support the pelvis by preventing downward and anterior rotation of the sacrum, thus contributing to the stability of the pelvic girdle. During pregnancy, especially in the later stages and childbirth, the pelvic ligaments including the sacrotuberous ligament become more relaxed to allow for the passage of the baby through the birth canal.