This hungarian composer published collections of folk music, based compositions on folk tunes, and developed a style in which he fused folk elements with highly developed techniques of art music more intimately than had ever done before.

The answer is Béla Bartók.

Explanation:

Béla Bartók is brilliant (as a graduate musician I have to tell you that, even though is not really part of the answer) Three Rondos on Slovak Folk Tunes, is a collection, amazing, by the way, of three small pieces for piano. It is very interesting to develop pianists playing techniques. His pieces are so complex, but really important, if you really want to master the instrument.

It is great to develop speed, listening, fingers strength, his pieces are the real workout and warm up for any pianist. And really develops your memory. If you are able to memorizes his complex pieces, than you can play anything. I am serious about this.

His unique style walk through the whole process of music learning. Every pianist should exercise with his books and learn music theory from him. The best pianist we have nowadays are all Béla Bartók learners.

all of them except "c"