Final answer:
The 's' in 'cats' serves as both a morpheme, which indicates the plural form of 'cat,' and a phoneme, representing the sound /s/.
Step-by-step explanation:
In the context of linguistics, phonemes and morphemes are essential building blocks of language. A phoneme is the smallest unit of sound that can distinguish one word from another in a particular language. A morpheme, on the other hand, is the smallest grammatical unit that has semantic meaning. For the word 'cats,' the 's' at the end is both a morpheme and a phoneme. As a phoneme, it represents the sound /s/, and as a morpheme, it signifies the plural form of 'cat'.