Which of the following sentences contains an adverb?

In the sentence "The children played happily in the park," the word "happily" functions as an adverb. Adverbs are words that modify verbs, adjectives, or other adverbs, typically providing additional information about how an action is performed. In this case, "happily" describes the manner in which the children played. It tells us more about the action of playing, indicating that they did so in a joyful or cheerful way.

This stands in contrast to the other sentences, where the words do not serve the adverbial purpose. For example, in "The flowers bloom in spring," the phrase "in spring" provides a temporal reference but does not modify the verb "bloom" directly in a way characteristic of an adverb. Similarly, "The sky is blue" and "The cat is on the roof" both focus on descriptions of the subjects and their states or locations, where there are no adverbs present that modify the verbs. Hence, "happily" is the only adverb in the given options.