Which of the following is NOT a characteristic of derivational morphemes?

Derivational morphemes are crucial in the formation of new words and the modification of root words. While they often change the meaning or part of speech of a word, it is not accurate to say that they always create a longer word. For example, adding the derivational morpheme "un-" to the root "happy" results in "unhappy," which is indeed longer, but there are cases where derivational morphemes can actually result in a shorter form, especially in irregular cases or with certain transformations (for instance, "ed" on "hit" results in "hit," which maintains the same form).

By contrast, derivational morphemes typically feature the first three choices: they can change a word’s part of speech (like turning "happy" into the noun "happiness"), they modify the meaning of the root (as "unhappy" conveys a meaning that is different from "happy"), and they can create entirely new words (for instance, "activate" from "active" or "happiness" from "happy"). Therefore, the choice that states they always create a longer word is not a defining characteristic of derivational morphemes.