During axial retention, the axis direction of the rotary instrument should be:

In the context of prosthetic dentistry, specifically during axial retention processes, having the axis direction of the rotary instrument parallel to the desired path of placement of the future restoration is essential. This alignment ensures that the preparation is optimally designed to facilitate the correct fit and secure retention of the restoration. By making the angle of the instrument parallel to this path, the practitioner can create contours in the tooth structure that enhance the stability and retention of the restoration once it is placed.

This approach also minimizes any potential obstructions during the insertion of the restoration, thereby improving the functional and aesthetic outcomes. Proper alignment supports the overall longevity of the restoration as it will allow for a more precise and secure fit, reducing the risk of loosening or failure over time.

The other options do not align with the fundamental principles of creating effective axial retention for restorations, as they describe orientations that either do not support this alignment or do not focus on the path necessary for the restoration to fit correctly.