What keyboard shortcut reveals the mask properties for the selected layer?

The keyboard shortcut that reveals the mask properties for the selected layer in Adobe After Effects is indeed M. When a layer with masks is selected, pressing the M key will display any mask attributes associated with that layer. This includes properties such as Mask Path, Mask Feather, Mask Opacity, and Mask Expansion, which are essential for refining the masking of the layer.

Using this shortcut is efficient for users who need to quickly access and edit mask properties without navigating through the layer’s dropdown menu. It allows for a more streamlined workflow, especially when making adjustments to visual elements in a project. This direct access is pivotal for tasks involving complex compositions where mask manipulation is frequently needed.

The other options provided do not correspond to the function of revealing mask properties in Adobe After Effects and would either invoke different commands or be non-functional in the context of mask visibility.