Final answer:
The ischial tuberosity is the primary bone that bears weight when sitting, forming part of the hip bone within the pelvis. The coccyx also receives some body weight in this position, but the ischial tuberosities are the main weight-bearing areas.
Step-by-step explanation:
The bone that bears weight when sitting is the ischial tuberosity, a large, roughened protuberance that forms the posteroinferior portion of the hip bone. When you sit, the upper body's weight is primarily supported by the pelvis, specifically the ischial tuberosities on each side. This structure is part of the pelvic girdle, which consists of the right and left hip bones, the sacrum, and the coccyx. The pelvis plays a critical role in not only supporting weight while sitting but also in transferring this weight to the lower limbs when standing, serving as an attachment point for muscles, and protecting internal pelvic organs.
Although the coccyx, or tailbone, does receive some body weight when sitting, it is the ischial tuberosity that is the primary weight-bearing region of the pelvis in a seated position. The pelvic girdle itself is designed to be a strong and stable structure to support the weight of the upper body and ensure proper locomotion by securely attaching the lower limbs to the axial skeleton.