What is required for a vehicle to be classified as 'self-propelled'?

For a vehicle to be classified as 'self-propelled', it must move under its own power. This means that the vehicle is equipped with its own source of propulsion, such as an engine or motor, allowing it to operate independently without being towed or pushed by another vehicle or external force. The essence of a self-propelled vehicle lies in its ability to generate the necessary motion autonomously.

The other choices may touch upon aspects related to self-propulsion, but they do not accurately define the term. A vehicle does not necessarily need to operate alone or without a driver to be considered self-propelled; rather, the main criterion is its capability to generate movement independently. Additionally, while a self-propelled vehicle might not rely on external power sources in some contexts, there are self-propelled vehicles, like electric cars, that indeed use electricity as a power source but still qualify as self-propelled. Thus, the clear and direct definition focuses solely on the vehicle's ability to move by its own power.