distance energy force time
I gave my students this multiple choice question: Jesus learned that a space probe named Voyager 1 was launched from Earth in 1977. It will take Voyager 1 more than 500 years to travel 1 light year. What is a light year used to measure in space?

Only one answer is correct among these 4 choices:
- distance
- energy
- force
- time

Could you tell me which one is correct?

distance