Which of the following is an example of a base word?

A base word is a word that stands alone and can form the foundation of other words. It carries the core meaning and can be modified by adding prefixes or suffixes to create new words.

In this context, "book" is the correct answer because it is a single, standalone word that does not have any prefixes or suffixes attached to it. It functions independently and serves as the base for related words, such as "bookish" or "booking."

The other options, such as "happiness," which contains the suffix "-ness," and "quickly," which contains the suffix "-ly," illustrate how additional endings can change the function or meaning of a base word. "Unkind" consists of the prefix "un-" added to the base word "kind," indicating the opposite meaning. Thus, "book" stands out as the only example that is a base word and can exist by itself without modification.