One ten-millionth of the distance from the equator to the North Pole
The wavelength of a spectral line of krypton 86
A platinum-iridium bar kept in Paris
The speed of light in vacuum, given the exact value 299 792 458 metres per second✓Ճիշտ պատասխան
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Because the speed of light is fixed by definition, the metre is the distance light travels in vacuum in 1/299 792 458 of a second. The other three answers are historical definitions that are no longer in force.