The correct answer is A. Morpheme
Step-by-step explanation:
In language, a morpheme refers to the basic unit in terms of meaning, this implies a morpheme is a letter or set of letters that might not be a complete word but still have a defined meaning together which understood by the speakers of a language. Additionally, morphemes can be classified as affix if they need from other morphemes to be words or roots if they are roots by themselves. For example, "dog" is a morpheme and root as this cannot be broken into small units but has complete meaning and "un" is an affix as this means not, but other morphemes need to be added to it to create words. Thus, the smallest unit of meaning is a morpheme.