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

The basic unit of length in the SI (International System of Units) system is the metre. The metre is defined as the distance light travels in a vacuum in a specific fraction of a second, making it a fundamental standard for measuring length. This unit is crucial in both scientific laboratories and daily life for providing a consistent framework for measurements.

The other options represent different types of measurements: newtons are used for force, cubic metres are a unit of volume, and kelvin is for temperature. Each of these has its own specific application and does not serve as a unit of length. Therefore, the metre stands out as the designated standard for length within the SI system.