Final answer:
The weight of the body when seated is supported by the ischial tuberosity, a part of the ischium in the posterolateral hip bone.
Step-by-step explanation:
When a person sits, the ischial tuberosity of the ischium supports the weight of the body. The ischium forms the posterolateral portion of the hip bone, which includes the expanded roughened area known as the ischial tuberosity. It is this portion that bears the weight when seated and can be felt against a chair when one wiggles the pelvis. Extending from the ischial tuberosity are other structures like the ischial ramus and the ischial spine, which play a role in muscle attachment and separating the sciatic notches, but it is the ischial tuberosity that primarily supports the body's weight when sitting.