Final answer:
The gerund phrase 'riding on the bus' serves as the direct object in the sentence, because it is the thing that 'we' enjoyed. Option D is the correct answer among the option provided in the question.
Step-by-step explanation:
The gerund phrase in the sentence 'We enjoyed riding on the bus.' is 'riding on the bus' Option D is correct. A gerund is a verb that ends in '-ing' and functions as a noun.
In this case, the gerund phrase is acting as the direct object of the verb 'enjoyed', because it is what is being enjoyed. Hence, the noun use of the gerund phrase 'riding on the bus' is as a direct object.
So Option D is the correct answer among the option provided in the question.