Light year is used to measure

The correct option is distance between stars.

Understanding Light Year:
A light-year is a unit of astronomical distance. It is defined as the distance that light travels in a vacuum in one Julian year (365.25 days).
Because light travels extremely fast, it can cover vast distances over the span of a year, making the light-year a useful unit for measuring the immense distances between stars, galaxies, and other celestial objects in the universe.

Mathematical Derivation of a Light Year:
We can calculate the physical distance of one light-year using the relationship:
Distance = Speed $\times$ Time

1. The speed of light in a vacuum ($c$) is approximately:
$c\approx 3\times {10}^8\text{m/s}$

2. The time ($t$) in one year (in seconds) is:
$t=365.25\text{days}\times 24\text{hours/day}\times 60\text{minutes/hour}\times 60\text{seconds/minute}$
$t\approx 3.156\times {10}^7\text{seconds}$

3. Calculating the distance ($d$):
$d=(3\times {10}^8\text{m/s})\times (3.156\times {10}^7\text{s})$
$d\approx 9.46\times {10}^{15}\text{meters}$ (or about $9.46\times {10}^{12}\text{kilometers}$)

Analysis of Options:
- distance between stars: Astronomical distances (like the distance between stars) are incredibly vast, making the light-year an appropriate unit of measurement.
- distance between atoms: Distances between atoms are microscopic and are typically measured in Angstroms (${10}^{-10}\text{m}$) or picometers (${10}^{-12}\text{m}$).
- stationary charge: Charge is a fundamental property of matter measured in Coulombs, not a distance.

Thus, a light-year is used to measure astronomical distances, such as the distance between stars.