What are affixes that attach to the beginning of words called?

Affixes that attach to the beginning of words are referred to as prefixes. This type of morpheme modifies the meaning of the base word or root to which it is attached, often indicating negation, position, time, or degree. For example, in the word "unhappy," the prefix "un-" alters the meaning of "happy" to express a negative connotation.

The other options, while related to affixes and morphemes, do not specifically refer to those that attach at the beginning. Suffixes attach at the end of words, inflectional morphemes modify a word to express different grammatical categories without changing the word's part of speech, and derivational morphemes can be prefixes or suffixes but are defined by their role in creating a new word or changing its form. Thus, the definition of prefixes aligns perfectly with the requirement of the question.