the same kind of cells working together. different systems working together. the same kind of organs forming a system. different organs performing a function.
I gave my students this multiple choice question: Heart muscle is best described as

Only one answer is correct among these 4 choices:
- the same kind of cells working together.
- different systems working together.
- the same kind of organs forming a system.
- different organs performing a function.

Could you tell me which one is correct?

the same kind of cells working together.