What is an essential characteristic of a bicycle according to traffic regulations?

An essential characteristic of a bicycle, as defined by traffic regulations, is that it must have at least two wheels. This definition is crucial because it distinguishes bicycles from other forms of transportation that may have varying configurations. Bicycles, by their nature, are designed to be stable and balanced when they have two wheels, which is a fundamental characteristic that allows them to function effectively as a mode of transport.

Having at least two wheels is important for safety and maneuverability, ensuring that the cyclist has the necessary support while riding. This definition aligns with how bicycles are categorized within traffic laws and other regulatory frameworks, reinforcing their role as a non-motorized vehicle designed primarily for transportation and recreation. Other options do not accurately reflect the fundamental design and purpose of bicycles within the context of traffic regulations, which is why they are not correct.