Which of the following is an example of a suffix?

A suffix is a word part that is added at the end of a root word to modify its meaning. The correct answer, which is the suffix "-ed," specifically indicates a grammatical function, usually signifying the past tense of a verb. For instance, when added to the verb "walk," it forms "walked," indicating that the action has already occurred.

The other choices represent prefixes, which are attached at the beginning of a word to alter its meaning. For example, "pre-" indicates "before," "un-" generally denotes negation or reversal, and "in-" can imply "not" or "into." These prefixes serve different functions but do not serve as suffixes, which is why they do not fit the description in the question.