A contra-angle matrix retainer may be indicated when restoring which of the following MN molar preparations?

A contra-angle matrix retainer is particularly useful in situations where there's a need for access to the preparation while also providing support for the matrix band, especially when the restoration involves a missing cusp. In the case of restoring a molar tooth with a missing distal buccal cusp, using a contra-angle matrix retainer allows the dental professional to efficiently position the matrix band around the remaining tooth structure. This ensures an appropriate contour and contact point can be established, which is essential for the functional and aesthetic outcome of the restoration.

Restore the tooth structure effectively when there is a significant loss of one cusp, like the distal buccal cusp in this scenario, presents unique challenges that a contra-angle retainer is specifically designed to handle. It provides the necessary adaptability and stability, allowing for more precise placement of the restorative materials. In contrast, the other scenarios listed involve different loss patterns or preparation depths that may not require the same level of specialized support from a contra-angle matrix retainer.