Which sentence has a gerund phrase that functions as a direct object?

AWalking along the Mercer River makes one feel at peace.
BIn the 1850s, preserving Yosemite National Park became a priority.
CGlacier Point is popular for providing spectacular views.
DRock climbers enjoy ascending the monolith El Capitan.

A gerund phrase is composed by a gerund (word ending in -ing) followed by a modifier, which are words that modify (adjectives or adverbs) or followed by objects. These type of phrases can be subjects or objects in a sentence, in the case of gerund phrases as objects it means it is the entity or object affected by the verb. In the sentence Rock climbers enjoy ascending the monolith El Capitan, the subject or person that carries our the action is Rock climbers; the verb or action is enjoy and the object or what is affected by the verb, it means what rock climbers enjoy is ascending the monolith El Capitan which is compound by a gerund (ascending) plus an object (the monolith El Capitan) which makes it a gerund phrase. Thus, the sentence that includes a gerund phrase is D. Rock climbers enjoy ascending the monolith El Capitan.