different substances mixed together atoms and molecules mixed together atoms of different elements combined together atoms of the same element combined together
I gave my students this multiple choice question: Which of the following defines a compound?

Only one answer is correct among these 4 choices:
- different substances mixed together
- atoms and molecules mixed together
- atoms of different elements combined together
- atoms of the same element combined together

Could you tell me which one is correct?

atoms of different elements combined together