Question

What makes the speed of light exactly 299,792,458 metres per second?

Answer 1

The definition of the metre.

Explanation:

The metre used to be defined as the length of a particular International Prototype Metre - a metal bar made from an alloy of platinum and iridium, measured at the melting point of ice.

In `1960`, the standard was changed to be expressed in terms of wavelengths of the red-orange spectral line of Krypton-86.

In `1983`, the definition was updated to be exactly `1/299792458` the distance travelled by light in vacuum in one second.