type of bait size of hook type of fish caught number of fish caught
I gave my students this multiple choice question: Robert is a fisherman who wants to find a way to catch more fish. He decided to try different sizes of hooks. Robert caught 4 catfish, 3 trout, and 7 perch while using worms for bait. Which is the independent (manipulated) variable in Robert's investigation?

Only one answer is correct among these 4 choices:
- type of bait
- size of hook
- type of fish caught
- number of fish caught

Could you tell me which one is correct?

size of hook