observation prediction inference hypothesis
I gave my students this multiple choice question: A student writes in a laboratory notebook: I placed a piece of iron in a beaker of water and the iron sank to the bottom of the beaker. What the student wrote in the laboratory notebook is an example of a(n)

Only one answer is correct among these 4 choices:
- observation
- prediction
- inference
- hypothesis

Could you tell me which one is correct?

observation