Which example correctly represents an infinitive?

An infinitive is the base form of a verb, typically preceded by the word "to." In this instance, "to run" is an example of an infinitive because it maintains the verb's action without being conjugated for a specific subject or tense. Infinitives can function as nouns, adjectives, or adverbs within a sentence. For example, in the sentence “I like to run,” "to run" acts as a noun, representing the concept of running.

The other choices represent different forms of the verb "run." "Running" is a gerund, which functions as a noun but is derived from a verb form that ends in 'ing.' "Ran" is the past tense of the verb, used to indicate an action completed in the past, while "runs" is the present tense form for third-person singular subjects. None of these options meet the criteria for an infinitive.