In linguistics, the smallest unit of sound that can distinguish one word from another is a phoneme.
A phoneme is an abstract sound category recognized as meaningful within a particular language or dialect. In English, changing the initial phoneme in cat from /k/ to /b/ produces bat, changing the word’s identity. Linguists write phonemes between slash marks, while square brackets are generally used for more detailed phonetic descriptions.
A phoneme is not necessarily one identical physical sound every time it is spoken. Different actual pronunciations, called phones, may belong to the same phoneme when they do not change a word’s meaning. These context-dependent variants are called allophones. For example, the /p/ in pin is typically pronounced with more aspiration than the /p/ in spin, but English speakers normally treat both as the same phoneme.
Phonemes are language-specific. A sound difference that distinguishes words in English may not distinguish words in another language. The spelling of a word also does not reliably reveal its phonemes: knot, not, and gnat have different spellings but similar consonant sounds. A grapheme belongs to writing, a syllable is a larger spoken unit, and a morpheme carries meaning, so none is the best answer here.