Final answer:
Luther's honk is an index as it indicates his presence to the other driver, providing a non-verbal alert to prevent an accident.
Step-by-step explanation:
When Luther honks his horn to alert another driver of his presence, the honk serves as an index. An index is something that indicates or points to the presence of a certain condition without using words or a verbal language. Unlike a symbol which is an arbitrary representation, or an icon which resembles what it represents, an index shows some causal or physical connection. In this case, the honk of the horn directly indicates the presence of Luther's car and his attention to prevent an accident.