Which of the following is not the unit of distance?

Answer: Option D

This question asks us to identify which option is not used to measure distance.
Let's look at each option:
Angstrom (Å): This is a very small unit of length, often used to measure the size of atoms and molecules. It's a unit of distance.
Light year (ly): This is a very large unit of distance. It represents the distance light travels in one year. It's a unit of distance.
Micron (µm): Also known as a micrometer, this is a unit of length equal to one millionth of a meter. It's used to measure small distances, making it a unit of distance.
Milestone: A milestone is a marker indicating distance along a road or path. It is not a unit of measurement itself; it's a physical marker showing a point in a distance.

Therefore, Milestone is not a unit of distance. The other options are all units used to measure length or distance.