Final answer:
The superior transverse axis of the sacrum is located at the upper part of the sacrum, specifically at the base of the sacrum, where it is thick and weight-bearing.
Step-by-step explanation:
The superior transverse axis of the sacrum is located at the upper part of the sacrum, specifically at the base of the sacrum. The sacrum is a triangular-shaped bone that is thick and wide at its superior base. This is the area where it is weight-bearing, and it then tapers down to an inferior, non-weight bearing apex.
The sacral promontory is the anterior lip of the superior base of the sacrum, which is a notable landmark in the region of the superior transverse axis. Furthermore, the superior articular processes located on either side of the superior opening of the sacral canal also indicate this region.
Therefore, the correct answer to the question is C. At the upper part of the sacrum.