When using the Mirror Feature command, which one of the following types of objects cannot be used as a Symmetry Plane?

In the context of using the Mirror Feature command, a non-planar face cannot be used as a symmetry plane because this command requires a flat, defined surface to create a mirrored feature accurately. The symmetry plane serves as a geometric reference that divides the object into two equal halves, allowing for a precise reflection of the geometry.

Planar faces, existing datum planes, and new datum planes all qualify as appropriate symmetry planes since they are flat surfaces. A planar face directly serves this purpose, while datum planes are specifically designed reference entities that can help maintain geometric relationships and facilitate complex design tasks, including mirroring features. Non-planar faces, on the other hand, lack the necessary uniform flatness, which makes it impossible to generate a symmetric counterpart reliably.