Which example indicates the function of an article in a sentence?

The example "A dog" effectively demonstrates the function of an article in a sentence because it includes the indefinite article "a," which introduces the noun "dog." Articles are used to provide context about the noun they modify, indicating whether the noun is specific or nonspecific. In this case, "A dog" suggests that it is any dog rather than a particular one that has already been mentioned or is known to the listener.

The other choices do not serve the function of an article. "Quickly running" is a phrase that primarily describes an action and lacks any articles. "And or but" consists of coordinating conjunctions that are used to connect words or phrases, and "In the park" is a prepositional phrase that provides location, but it does not contain an article modifying a noun. Thus, "A dog" clearly illustrates the role of an article in a sentence structure, making it the correct choice.