What is a characteristic of a word that cannot have a combining form?

The correct choice highlights that a word which cannot have a combining form is characterized by its inability to accommodate a combining form at the end. A combining form is a root word that has been modified to accept affixes like suffixes or prefixes, typically seen in medical terminology, where roots often combine with vowels for better pronunciation.

When a word cannot accept a combining form, it means that it has a structure or ending that does not permit the addition of further morphological elements, which restricts it from being altered or transformed into a more complex form.

The other options do not accurately capture the essence of the defining characteristics of such words. For instance, a word that ends with a suffix may still allow for a combining form, and having a prefix or beginning with a root does not inherently prevent a word from utilizing a combining form. The essence of this characteristic resides in the structural limitations imposed by the word itself.