Explanation:

- Option 1: Reliable: This word is spelled correctly. It means dependable or trustworthy.

- Option 2: Realm: This word is also spelled correctly. It refers to a kingdom or a field of activity or interest.

- Option 3: Resemble: This word is spelled correctly and means to look like or be similar to something or someone.

- Option 4: Redius: This is the incorrectly spelled word. The correct spelling should be "Radius," which is a term often used in mathematics to describe the distance from the center of a circle to any point on its edge.

Answer: Option 4 - Redius