Which word from the diagram below is a preposition?

The word "for" is indeed a preposition, which is a part of speech that typically introduces a prepositional phrase and shows the relationship between a noun or pronoun and other words in a sentence. Prepositions often indicate direction, location, time, or method. In this case, "for" can express purpose or benefit, as in phrases like "for you" or "for a reason."

Other choices represent different parts of speech. "Football" is a noun that refers to a type of sport or the ball used in the game. "Yesterday" is an adverb that specifies a time reference in the past. "Monday" is also a noun, specifically a day of the week. Understanding the function of "for" helps identify it as a preposition in contrast to these other parts of speech.