What is the term for a prefix that changes its final letter according to the base it attaches to?

The term for a prefix that changes its final letter according to the base it attaches to is known as an assimilated prefix. This type of prefix undergoes a phonetic change to merge smoothly with the base word to which it is affixed, enhancing the transition between the prefix and the root. For example, the prefix "ad-" which can become "ac-" before a base word starting with "c" or "k," as in "accompany."

The other terms listed do not accurately describe this feature. A base word refers to the main part of a word that carries meaning and is often what prefixes and suffixes attach to. A root word is the basic form of a word without any affixes, while a morpheme is a broader term referring to the smallest unit of meaning in a language, which can be a word, part of a word, or a prefix or suffix. Hence, "assimilated prefix" is the most precise term for the concept being described.