Which of the following best defines a suffix?

A suffix is defined as a word part that is added to the end of a root word to alter its meaning or create a new word. This modification can change the meaning of the word significantly and can also affect its grammatical function within a sentence, such as turning a verb into a noun or changing the tense of a verb.

For example, adding the suffix "-ly" to the adjective "quick" results in the adverb "quickly," which modifies how the action is performed. In this way, the suffix not only changes the meaning but also enhances the word's function within the context of a sentence.

The other options do not fully capture what a suffix does: the beginning of a word refers to a prefix, expressing a tense more directly pertains to certain suffixes but does not encompass all suffixes, and a word part that has no meaning by itself typically describes a morpheme, not specifically a suffix. Thus, the correct understanding of a suffix lies in its ability to change or modify the meaning of the original word.