What feature distinguishes a consonant from a vowel in syllables?

The distinguishing feature of consonants and vowels in syllables is that consonants require a vowel for syllable formation. Every syllable in the phonetic structure typically consists of at least one vowel sound, known as the nucleus, which is the core element. Consonants can serve as onsets, which occur before the vowel, or as codas, which come after the vowel, but they cannot stand alone as a syllable without a vowel present. This necessity for a vowel to create a syllable is what sets consonants apart from vowels, which can function independently as syllables, such as in the examples of "a" or "I."

The other features mentioned do not serve as distinguishing characteristics between consonants and vowels in the context of syllable formation. While some consonants can be voiced, not all are, and vowels are not strictly 'always voiced.' The ability to occur alone as a syllable is a feature characteristic of vowels, counter to what is posed in the choices. Finally, consonants do not produce sound without airflow; they typically involve some form of regulated airflow, while vowels are produced with a more open vocal tract.