Which command displays a marker at the center of mass within a selected object?

The command that displays a marker at the center of mass within a selected object is the one labeled 'Center of Mass.' This command is specifically designed to calculate and visually represent the centroid of an object based on its geometry and material properties. By utilizing this command, users can accurately pinpoint the center of mass, which is crucial for tasks involving balance, stability analysis, and structural design.

In contrast, the other options do not serve the same purpose. For example, 'Centerline' is commonly used to create reference lines within a drawing but does not relate to mass or physical attributes of the object. Similarly, 'Mass Center' and 'Structural Center' are terms that may appear similar, but they either refer to different concepts in structural analysis or might not be recognized commands in the context of displaying a marker for the center of mass specifically. This makes 'Center of Mass' the most accurate and relevant choice for achieving the desired outcome in a design or engineering context.