What is a "suffix"?

A suffix is a letter or group of letters added at the end of a word to change its meaning or grammatical function. This addition can alter the tense, number, or part of speech of the base word. For example, adding the suffix "-ing" to the verb "run" creates "running," indicating a present action, while adding "-ly" to the adjective "quick" transforms it into the adverb "quickly," showing how an action is performed. The definition encompasses how suffixes expand and modify the language, making option A the most accurate definition in this context.

The other choices do not accurately capture the essence of what a suffix is; a word that stands on its own describes a complete lexical item, while a related term discussing concepts or a synonym would refer to different aspects of language, not the structural element of attaching letters to a base word for modification purposes.