The length of a meter is defined based on which phenomenon?

The correct answer, which defines the length of a meter, is based on the distance light travels in a defined time interval. This definition is rooted in the international standard set in 1983, when the meter was defined as the distance that light travels in a vacuum in 1/299,792,458 of a second. This approach provides a highly precise and reproducible standard, given that the speed of light in a vacuum is a constant of nature.

This definitional method relies on the universal nature of the speed of light, making it an ideal reference for accuracy in measurements. Other methods, such as the vibration of a quartz crystal, while useful for practical measurement devices, do not provide the same level of precision and universality as the speed of light in a vacuum. Similarly, using the circumference of the Earth or the speed of sound in air would be subject to variability based on environmental conditions and would not be as universally applicable or accurate.