What is the key distinguishing feature of a suffix?

A suffix is specifically defined as a morpheme that is added to the end of a word to modify its meaning or to create a new word, establishing its key distinguishing feature as appearing at the end of a word. For example, in the word "happiness," the suffix "-ness" is added to the root word "happy" to form a noun that represents the quality of being happy. This modification often involves changes in the grammatical category of the word, such as turning an adjective into a noun. While it's true that suffixes can also influence word class, the defining characteristic is their position at the end of the word rather than their ability to change classes or their length. Other options incorrectly describe the characteristics of morphemes; for instance, prefixes occur at the beginning of words, and suffixes can vary in length beyond just a single letter.