Which of the following sentences contains a gerund?

A gerund is a verb form that ends in -ing and functions as a noun in a sentence. In the provided sentence, "She enjoys reading novels," the word "reading" acts as a noun. It is the object of the verb "enjoys," indicating what she enjoys doing. The structure of the sentence showcases this usage clearly, as "reading novels" describes an activity that is enjoyed, thus making "reading" a gerund.

In contrast, the other sentences do not contain gerunds. For instance, in "He is running a marathon," "running" is part of a verb phrase indicating an action, rather than functioning as a noun. "They will complete the project" includes no gerunds at all, as "complete" is used as a verb. Lastly, in "We walked to the park," "walked" is again a verb in the past tense, not serving as a noun. This distinction is crucial for understanding the function of gerunds within sentences.