Which term is commonly used to refer to words formed without the use of prefixes or suffixes?

The term that refers to words formed without the use of prefixes or suffixes is indeed a base word. A base word stands as the simplest form of a word, to which affixes can be added to create new words or vary its meaning. For example, "play" is a base word, while "replay" and "playing" would include prefixes or suffixes, respectively.

Other terms in the choices relate to different aspects of word formation. "Constructed word" implies a word that is made up, usually referring to a word that is intentionally created or fabricated, which is not the same as a base word. A "morpheme" is a more general linguistic term that refers to the smallest unit of meaning in a language; this includes base words, prefixes, and suffixes, so it does not specifically indicate a word without affixes. Lastly, "complex word" usually refers to words that consist of a base word plus one or more affixes, thus further distinguishing it from a base word.