What is a defining characteristic of a moped?

A defining characteristic of a moped is that it does not have an external shifting device, which differentiates it from motorcycles or scooters that do have gears or manual transmission systems. Mopeds are typically designed for ease of use, often featuring automatic transmissions that allow the rider to operate the vehicle without worrying about shifting gears manually. This characteristic contributes to their classification and is significant in understanding the operational dynamics of mopeds as compared to other types of motorized two-wheelers.

The other options, while discussing aspects that can be found in various vehicles, do not accurately represent a moped's definition. For instance, having four wheels would classify a vehicle as something other than a moped, typically a car or a quad. The mention of exceeding a motor size of 50cc is incorrect for mopeds, as they are generally limited to this engine size or lower. Lastly, the maximum speed of 40 mph does not universally apply to all mopeds, as speed can vary; however, the lack of an external shifting device remains a crucial signal in identifying a moped.