The smallest unit of sound that distinguishes on word from another in a particular language is_____________.

Correct option is A

Phoneme
The smallest unit of sound that distinguishes one word from another in a particular language is a Phoneme. Phonemes are the basic vocal gestures of a language, the smallest contrastive linguistic unit which may bring about a change of meaning.
• For example, in English, the "p" in "pat" and the "b" in "bat" represent two different phonemes. A morpheme, on the other hand, is the smallest grammatical unit in a language, syntax refers to the arrangement of words and phrases to create well-formed sentences, and semantics is concerned with meaning.