Question

A style of singing in which the vocalist sings nonsense syllables is called

Answer 1

A style of singing in which the vocalist stings nonsense syllables is called a scat so B

Answer 2

Scat. It's a form of jazz expression where the singer is almost imitating a musical instrument's improvisational solo (a solo which is created on the spot). See: https://youtu.be/2xoSFyqW7JI