a stone rolling downhill a vehicle traveling on a bumpy road a string vibrating on a guitar a grasshopper jumping up and down occasionally
I gave my students this multiple choice question: Which of the following is the best example of a wave?

Only one answer is correct among these 4 choices:
- a stone rolling downhill
- a vehicle traveling on a bumpy road
- a string vibrating on a guitar
- a grasshopper jumping up and down occasionally

Could you tell me which one is correct?

a string vibrating on a guitar