A moped is defined by which of the following characteristics?

A moped is defined primarily by its engine size and transmission characteristics. Specifically, it typically features an engine that does not exceed 100cc piston displacement, which is a defining characteristic of mopeds. This limited engine size ensures that mopeds are less powerful and generally safer for urban commuting and novice riders.

Additionally, mopeds are usually designed without a gear shift, emphasizing ease of use and accessibility. This characteristic allows for a simpler riding experience, as the rider does not need to manually change gears.

Therefore, the combination of these two characteristics—engine size and the absence of a gear shift—accurately encapsulates the definition of a moped, making the answer that includes both conditions the correct choice. The other options do not accurately reflect the legal and functional specifics that define a moped, such as the four-wheel attribute, which would classify the vehicle differently.