Final answer:
The posterior sacroiliac ligaments are responsible for resisting the posterior rock of the sacral apex during upright posture, providing crucial support to the sacroiliac joint.
Step-by-step explanation:
The ligaments that resist the posterior rock of the sacral apex during upright posture are the posterior sacroiliac ligaments. These are a pair of strong ligaments that span the sacrum and the ilium of the hip bone, providing support to the posterior side of the sacroiliac joint. This support is crucial for maintaining stability and resisting the forces that attempt to push the apex of the sacrum backward during the upright posture of the human body.