Which statement is true about a moped?

A moped is specifically defined in regulations based on its engine size, and a critical aspect of this definition is the limitation on piston displacement. A moped must have a piston displacement of 50 cubic centimeters (cc) or less, which establishes it as a low-powered vehicle intended for use on public roads with certain restrictions.

While it might be possible for some mopeds to reach speeds over 30 miles per hour, this is generally not a common characteristic of traditional mopeds, as they are designed for lower speeds primarily for safety and ease of use. The ability to shift gears can sometimes be a feature in models classified as mopeds, but it's not a defining characteristic, as many mopeds are geared towards simplicity and typically have a single gear. Additionally, stating that a moped is a motorized bike with no power drive system is misleading since mopeds are designed with an engine that provides the necessary power for operation, making this statement inaccurate.

Thus, the defining feature that correctly identifies a moped is its limitation on piston displacement, making the statement regarding piston size the accurate choice.