What is added to a root word to change its meaning?

The correct answer is a prefix because a prefix is an affix that is added to the beginning of a root word to modify its meaning. For example, when "un-" is added to the root word "happy," it changes the meaning to "not happy." This demonstrates how prefixes work to alter the original meaning of a word, often indicating negation, place, time, or number.

Understanding the role of prefixes is crucial in expanding vocabulary and enhancing comprehension, as they frequently convey significant information about the word's meaning.