The complete sentence after filling the blanks with an adverb will be, ''The Oak Alley Hotel offers frequent-traveler discounts everywhere in its hotel network to the guests who use the hotel more than 7 nights a year.'' Therefore, the option C holds true.
An adverb is regarded as a word of a phrase, which is helpful in modification of a noun or a verb in a sentence in order to show the relation of the word with respect to place, time, manner, cause etc. with the noun or verb.
In the given sentence, using the adverb everywhere helps in showing the relation of the Oak Alley Hotel and the discount it offers to its customers in its entire hotel network. Using 'everywhere' as an adverb makes the sentence more meaningful.
Therefore, the option C holds true regarding the significance of an adverb in a sentence.
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