What does the suffix "er" indicate when added to a root word?

The suffix "er" typically indicates a person who performs an action, which is why the choice indicating this is correct. For example, in words like "teacher" or "runner," the suffix "er" is added to the root words "teach" and "run," respectively, effectively transforming those verbs into nouns that denote someone who engages in those actions. This formation helps to create clear connections between the action and the individual performing it, which is fundamental in language for specifying roles or jobs related to various activities.

In contrast to this, the other choices do not correctly represent the function of the suffix "er." The quality of something generally corresponds to different suffixes, such as "ness" or "ity." The act of doing something aligns more with the base or root form of a verb rather than the noun form represented by "er." Finally, negating the action of the root word is typically accomplished by prefixes like "un-" or "non-," rather than through the use of "er."