What do we call a piece of scenery that is mounted on wheels for easy movement on stage?

The term for a piece of scenery that is mounted on wheels for easy movement on stage is "truck." In theatre, a truck allows for efficient scene changes by enabling large set pieces to be moved on and off stage quickly and smoothly. This is especially useful in productions where rapid transitions are necessary or where space is limited. Trucks can be designed to carry a variety of scenic elements, making them versatile tools in stagecraft.

In contrast, a platform typically refers to a raised stage section that remains fixed in place, a backdrop serves as a stationary background for scenes, and a scenic piece can denote any decorative element used in the set, not necessarily one that is movable. Hence, the specificity of the term "truck" aligns perfectly with the definition of a wheeled scenery piece, making it the correct choice.