What are open and voiced sounds produced by the easy passage of air through a relatively open vocal tract known as?

The correct choice is vowels. Vowels are characterized by a relatively open vocal tract, which allows for the free passage of air. This results in sounds that are typically voiced, meaning that the vocal cords vibrate during their production. Vowels are essential elements of speech and serve crucial roles in language structure, including forming the nucleus of syllables and contributing to word formation.

In contrast, consonants are produced with some degree of constriction or closure within the vocal tract, which creates various articulatory qualities distinct from vowels. Syllables are units of sound that may include vowels, consonants, and sometimes other sounds, but they do not define the characteristics of sound production. Onsets refer specifically to the initial consonant or consonant cluster of a syllable, further differentiating them from the qualities of vowel sounds.