What is the basic unit of length in the metric system?

In the metric system, the basic unit of length is the meter. This unit serves as the foundational measurement from which other units of length are derived. For instance, one centimeter is one hundredth of a meter, and one kilometer is one thousand meters. The meter is defined based on the distance light travels in a vacuum in a specific fraction of a second, which offers a standardized and universal measurement that can be applied in scientific contexts.

The other options, such as centimeter and kilometer, are both metric units but are derived from the meter rather than being the foundational unit themselves. The mile, on the other hand, is part of the imperial system, not the metric system, making it an unsuitable choice when identifying the basic unit of length in the metric system. Thus, the meter's designation as the primary unit underlines its essential role in metric measurements.