The correct answer is D. to ride the bus
Explanation:
In grammar, an infinitive refers to a basic verb form that includes the preposition "to" followed by the bare verb; for example, to jump, to run or to agree. Additionally, an infinitive phrase refers to a set of words that begin with an infinitive and include other words that complement the meaning of the infinitive. These phrases are used in the sentence as direct objects, adjectives or even adverbs.
According to this, in the sentence "Too many people drive to work when it would be just as easy for them to ride the bus" the infinitive phrase is "to ride a bus" because this is the section of the sentence that includes an infinitive "to ride" and other words "the bus" that complete the meaning of the infinitive.
