What is a morpheme?

A morpheme is indeed defined as the smallest meaningful unit of language, making this choice accurate. Morphemes can either function as standalone words, like 'cat' or 'tree,' or act as prefixes or suffixes that modify the meanings of base words, such as 'un-' in 'undo,' which alters the root word's meaning. Understanding morphemes is crucial in linguistics, as they are the foundational building blocks of words and contribute to meaning in language.

The other choices represent different linguistic concepts. The smallest sound unit in speech refers to phonemes, not morphemes. A word that conveys a complete thought relates to a sentence or clause, which may include multiple morphemes. Lastly, a grammatical structure pertains to the rules that govern how words and phrases are arranged in language, which is a broader concept than a single morpheme.