Final answer:
In the sentence provided, 'fruit' serves as a noun modifying the noun 'salad' to describe the type of salad, hence functioning as an adjective, also known as an attributive noun.
Step-by-step explanation:
The noun used as an adjective in the sentence 'Omar had fruit salad at lunch' is fruit. It is functioning as an adjective because it describes the kind of salad Omar had by specifying the ingredients in the salad, which in this case, are fruits. In English, nouns can sometimes act as adjectives to modify other nouns, a role often referred to as a noun modifier or attributive noun.