Final answer:
The word 'Tomorrow' functions as an adverb in the given sentence by modifying the verb 'will tour' and indicating when the action of touring the reptile zoo is set to occur.
Step-by-step explanation:
The best answer to identify the part of speech of the boldfaced word 'TOMORROW' in the sentence "Tomorrow, we will tour the reptile zoo" is adverb. This is because 'tomorrow' is modifying the verb 'will tour', indicating when the action will take place. In English grammar, words that modify verbs, adjectives, or other adverbs are classified as adverbs. They often answer questions such as how, when, where, to what extent, or under what condition something happens. In contrast, adjectives describe or modify nouns and pronouns, subjects act as the doers of the action in a sentence, and indirect objects receive the action of a transitive verb indirectly.