When you want to find the meaning, synonym, or other information about a word, you use the phrase "Look up." This is a common phrase used when referring to searching for something specific in a reference source, like a dictionary or thesaurus. Here, you are being asked to look up the word in the Thesaurus to find more information about it, such as its synonyms. Thus, the correct completion of the sentence is to "Look up the word in the Thesaurus!"