Arkansas Foundations of Reading Test (FORT) 2025 - Free FORT Practice Questions and Study Guide

The best definition of a prefix is that it is a syllable or word that comes before a root word to change its meaning. Prefixes are specific types of affixes that attach to the beginning of a root or base word, altering its meaning by adding a new layer of significance. For example, the prefix "un-" in "unhappy" indicates a negation of the word "happy," thereby changing its meaning to express the opposite sentiment.

Other options provided do not accurately describe a prefix. A definition that discusses a word modifying a verb pertains more to adverbs or adjectives rather than prefixes. The option concerning the main part of a word aligns with the definition of a root word, which stays central to the word's basic meaning but does not involve the modification that prefixes accomplish. Lastly, a description of letters added at the end of a word refers to a suffix, which is distinct from the concept of a prefix. Thus, the definition highlighting the prefix’s role in modifying the meaning of a root word is the most accurate choice.