Answer:
3) nouns
Step-by-step explanation:
Morphemes are the smallest linguistic unit within a word that has meaning, and inflectional morphemes or suffixes refer to those morphemes added at the end of a word that completes the whole meaning of the word.
In English, plural inflectional morpheme endings can only be added to nouns since are the nouns the only part of speech that can change from singular to plural by adding a suffix. For example, car - cars, tea - teas, microscope - microscopes, glass - glasses.