Question

Question #f25e6

Answer 1

The SI unit of measurement is the meter (symbol `"m"`)

Explanation:

By fundamental I'll assume you mean something along the lines of "most used" or "most basic" or "most common".

Essentially the answer to all three of these is the meter (`"m"`), which is one of the base SI units of measurement.

The meter is defined as the distance light travels in vacuum in a time period of `1/(299,792,458)` `"second"`.

This value comes from the speed of light in vacuum, defined precisely as `299,792,458` `"m/s"`.