Explanation:

- The SI unit of length is a fundamental measurement in the International System of Units.

- Option 1: Metre

- The metre is the official SI unit of length.

- It is widely used globally.

- Option 2: Millimetre

- It is a derived unit of length.

- There are 1,000 millimetres in a metre.

- Option 3: Centimetre

- Also a derived unit.

- There are 100 centimetres in a metre.

- Option 4: Kilometre

- Also a derived unit.

- There are 1,000 metres in a kilometre.