Final answer:
The sacrotuberous ligament extends from the PSIS and the lower part of the sacrum and coccyx to the ischial tuberosity.
Step-by-step explanation:
The ligament that extends from the posterior superior iliac spine (PSIS) and the lower part of the sacrum and coccyx to the ischial tuberosity is the B. Sacrotuberous ligament. This ligament is one of the key structures that helps to support and stabilize the pelvis. It also contributes to the formation of the greater and lesser sciatic foramina, which are important passageways for neurovascular structures exiting the pelvis.