What is the term for one or more letters attached to the beginning, end, or base of a word to create a derivative word or an inflectional form?

The term for one or more letters attached to the beginning, end, or base of a word to create a derivative word or an inflectional form is "affix." An affix is a general term that encompasses both prefixes (which are added to the beginning of a root word) and suffixes (which are added to the end of a root word). It refers to any element that modifies the base word to change its meaning or grammatical function.

While "prefix" specifically refers to letters added to the front of a word, and "suffix" refers to those added to the end, "affix" includes both types as well as any other forms that may modify the root. Therefore, "affix" is the broader term that accurately represents this linguistic concept, making it the correct answer to the question. The other terms are more specific and do not capture the full range of meaning that "affix" does.