Let's examine the four answer choices that you are given.
A.) falsetto signing
Falsetto means to sing in a voice that is higher than a singer's normal range. It is commonly used for male singers, especially tenors, however, all singers can use a falsetto voice.
B.) scat singing
Listen to most jazz performances and you'll hear scat singing. When vocalists are scatting, they're making up words to go along with the beat of the music. It's kind of like they're imitating an instrument like a drum beat or guitar note.
C.) call and response singing
Think of the song "Down By the Bay" you may have sung when you were younger. This is an example of a call and response song because someone sings a line or plays music, then another person (or group) will respond back with another line.
D.) melodic improvisation
This is making up music in the moment - just as you're feeling the song, you start to play notes or sing along. Sometimes musicians take pieces that are familiar and add parts as they perform. This occurs often in live performances.
Looking back at your answer choices, the best one to describe improvised singing/music with nonsense syllables would be B.
