Step 1: Define linguistic units.
A morpheme is the smallest linguistic unit that carries meaning, such as roots, prefixes, or suffixes.
Step 2: Distinguish from phonemes.
Phonemes are the smallest units of sound but do not carry meaning on their own.
Step 3: Eliminate incorrect options.
Syntax and semantics are branches of linguistics, not units of speech perception.
Step 4: Conclusion.
Therefore, the smallest meaningful unit of speech perception is a morpheme.