What is defined as a morpheme that modifies the meaning of a root word?

The correct answer is a prefix. A prefix is a type of morpheme that is added to the beginning of a root word to modify its meaning. For example, the prefix "un-" can be added to the root word "happy" to create "unhappy," which changes the meaning to indicate the opposite. Prefixes are essential in language as they provide additional context or a change in meaning without altering the base form of the word itself.

While suffixes, which are morphemes added to the end of a root word, also modify meanings, the question specifically asks for a morpheme that appears at the beginning. Root words serve as the foundational words without any modifications. Compounds are formed by combining two or more root words, and do not necessarily involve modification in the same way that prefixes or suffixes do. Thus, understanding prefixes is crucial for grasping how they interact with root words to form new meanings.