Which One of the Following Children Is Definitely Working in His

Which one of the following children is definitely working in his or her zone of proximal development?

A) Arnold uses correct grammar and punctuation when he writes short stories.
B) Berta is beginning to learn basic woodworking techniques. She has trouble hammering a nail straight into a piece of wood unless her teacher stands beside her, helping her and reminding her of what to do.
C) Calvin is playing the clarinet in the band. He finds that he can more easily keep the appropriate tempo if he taps the beat with his foot.
D) Doreen finds it virtually impossible to solve mathematical word problems, even when her teacher gives her helpful hints.