Which of the following words correctly completes the sentence: "She sings____________."?

The correct word that completes the sentence "She sings____________" is "beautifully." This word is an adverb that describes how she sings, indicating that she sings in a beautiful manner. Adverbs often modify verbs, adjectives, or other adverbs, and in this case, "beautifully" enhances the verb "sings."

The word "beautifully" is derived from the adjective "beautiful" by adding the adverbial suffix "-ly," which is a common way to form adverbs in English. Using this structure allows the sentence to clearly convey the intended meaning with proper grammatical form.

Other choices do not conform to standard English usage. "Beautifuly" is a misspelling and lacks the necessary "l" to be correctly formed. "Beautyfully" is also an incorrect spelling; the correct adverbial form is "beautifully." Lastly, "Beautifly" is a misspelling that incorrectly suggests an unrelated word, thus rendering it unsuitable. Therefore, "beautifully" is the only choice that appropriately fits the context and grammatical requirements of the sentence.