Proximal retention grooves in a proximal-occlusal silver amalgam restoration are most necessary when the occlusal extension is:

Proximal retention grooves in a proximal-occlusal silver amalgam restoration are important for maintaining the structural integrity and longevity of the restoration. These grooves are particularly necessary when the occlusal extension is narrow faciolingually relative to the proximal extension. This is because in such cases, there is less mechanical retention provided by the restoration and the grooves help to increase the surface area for bonding and improve retention. Therefore, the other options (B Wide mesiodistally, C: Shallow, and D: Deep) are incorrect as they do not address the specific need for proximal retention grooves in this type of restoration.