Explanation:

- "Up in arms" typically refers to people being very upset or angry, often implying they're ready to protest or defend their views.

- Option 1: In rebellion

- This is the correct interpretation. "Up in arms" suggests people are rebelling or protesting against something.

- Option 2: Begin fighting

- Although it sounds similar, "up in arms" is more about being ready to fight (often metaphorically) rather than actually starting a physical fight.

- Option 3: Raise something

- This option doesn't fit the idiom. It might suggest lifting or elevating something physically.

- Option 4: In unity

- While people can be united in protest, this option doesn't capture the element of anger or opposition implied by "up in arms."