In the word "runner," what does the suffix "-er" represent?

In the word "runner," the suffix "-er" functions as a derivational morpheme that alters the meaning of the base word "run." Specifically, it changes the verb "run" into a noun that denotes a person who performs the action of running. This addition of "-er" effectively transforms the word from an action to a specific identifier for someone associated with that action, thus illustrating how derivational morphemes create new words and expand the lexicon by modifying the grammatical category and meaning of the original word.

The other options do not apply because the formation of "runner" does not involve a combination of two independent words, which would be the case in compounding. Code-switching refers to alternating between languages or dialects within a conversation, which is unrelated here. Semantic broadening describes how a word's meaning expands over time to include more concepts, which is also not the role of "-er" in "runner."