What is defined as the distance that light travels in a vacuum in 1/299792458 of a second?

The definition provided in the question refers specifically to the metre. In the International System of Units (SI), the metre is defined as the distance light travels in a vacuum in precisely 1/299792458 of a second. This definition integrates the speed of light into the measurement of distance, thereby establishing the metre as a fundamental unit based on a universal constant.

Understanding this definition is essential because it illustrates how the concept of distance is closely tied to the speed of light, which is a critical constant in physics. While a light-year is also a measure of distance that is based on the speed of light, it corresponds to the distance light travels in one year, not in a fraction of a second. The kilometre, being a larger unit of distance, is defined as 1,000 metres, and a second is a unit of time, not distance. Thus, this context reinforces why the metre is the correct answer.