The unit used for measuring length is:

The correct answer is option 'D' - Metre.

Explanation:

Length and Measurement:
Length is a physical quantity that refers to the measurement of something from one end to another. It is one of the fundamental quantities in physics and is used to describe the size or extent of an object or distance between two points.

The Metric System:
The metric system is an internationally recognized decimal-based system of measurement that is widely used around the world. It is based on units of 10, making it easy to convert between different units.

The Metric Unit for Length:
The metric unit used for measuring length is the metre (m). The metre is defined as the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second. This definition ensures that the length of a metre remains constant and can be replicated accurately anywhere in the world.

Subunits of the Metre:
The metre can be divided into smaller units or multiplied into larger units to measure lengths of different magnitudes. Some common subunits and multiples of the metre include:

- Centimetre (cm): 1 metre = 100 centimetres
- Millimetre (mm): 1 metre = 1000 millimetres
- Kilometre (km): 1 kilometre = 1000 metres

Other Units of Length:
While the metre is the standard unit for length in the metric system, there are other units used in different systems of measurement. Some examples include:

- Inch (in) and foot (ft): These units are commonly used in the Imperial system, primarily in the United States and a few other countries.
- Yard (yd) and mile (mi): These units are also part of the Imperial system and are commonly used for longer distances.

Conclusion:
In summary, the unit used for measuring length is the metre (m) in the metric system. The metre is a fundamental unit that can be subdivided or multiplied to measure lengths of different sizes accurately. While there are other units of length used in different systems, the metre is the most widely recognized and used unit for measuring length worldwide.