Which of the following describes a prefix?

A prefix is defined as an element added to the beginning of a root word, which modifies the meaning of the root. For example, in the word "unhappy," "un-" is a prefix that negates the meaning of "happy." This usage illustrates how prefixes alter the base word, creating a new meaning while maintaining the core of the original root.

The other options relate to different linguistic concepts. Adding a word to the end of a root describes a suffix, while a root word without modifications simply refers to the word in its pure form. A compound word consists of two or more root words combined, which does not accurately represent the function of a prefix. Thus, recognizing that a prefix specifically enhances or changes the meaning of a root at the beginning establishes why this choice is correct.