Which modern language is the only surviving representative of Bolgar Turkic?

The Chuvash language is the only modern surviving representative of the Bolgar Turkic branch of the Turkic languages family.12

Originating from the extinct Volga-Bolgarian language, Chuvash is distinct from the Common Turkic languages, exhibiting archaic features and a lack of mutual intelligibility with other Turkic tongues. While Common Turkic and Bolgar Turkic diverged early in their history, Chuvash remains the sole descendant of the Bolgar type.12