Explanation:

- The SI unit for length is actually the metre (written as "meter" in American English). 

- Here’s what that means: when scientists or anyone working internationally needs to measure how long something is, the metre is the baseline standard.

- Centimetre (option 2) and millimetre (option 3) are both units of length, but they’re smaller divisions of the metre. One metre = 100 centimetres, and one metre = 1,000 millimetres.

- Kilometre (option 4) is also a unit of length, just much bigger—a kilometre is 1,000 metres.

- So, for “official” scientific and worldwide use, the metre is the cornerstone for measuring length.

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