Which word from the diagram is a noun?

The word that is a noun in this context is football. A noun is a part of speech that represents a person, place, thing, or idea. In this case, "football" refers to a specific sport and is clearly a thing, which fits the definition of a noun perfectly.

While "yesterday" refers to a time and can act as a noun in certain contexts, it often denotes an abstract concept rather than a tangible thing. The word "for" is a preposition, used to indicate relationships between other words in a sentence. "Monday," on the other hand, is also a noun indicating a specific day of the week, classifying it similarly to "football." However, since the query is looking explicitly for a type of noun in the context provided, focusing on "football" directly relates to the concept of a person participating in the sport, an object involved in an action, or a subject of discussion.