Solution
✅ Correct Option: 4
Rare -> something uncommon or special.
- In sentence I, it's used correctly to describe a special coin.
- In sentence II, "rare" is used correctly to describe an uncommon type of cancer.
- In sentence III, "rare" is wrong - it should be "rear" which means the back of something.
Note: "rare" = uncommon/special, while "rear" = back/behind.