Which part of speech is represented by the word in parentheses in the sentence: "I called you (on) Monday?"

The word "on" in the sentence "I called you (on) Monday" functions as a preposition. Prepositions are words that link nouns, pronouns, or phrases to other words within a sentence, typically indicating relationships in terms of time, space, or direction. In this case, "on" connects the noun "Monday" to the action of calling, specifying the time when the action occurred.

Understanding the role of "on" as a preposition helps clarify the overall structure of the sentence, showing how it conveys a specific timeframe for the action. The prepositional phrase "on Monday" informs the reader about when the calling took place, highlighting the importance of prepositions in providing contextual information within sentences.