The best answer is that concertos (which are large bodies of compositional work) were written for professional musicians who commissioned them. So the answer is A. Concerto's are large pieces of work often involving a solo instrument and full orchestra backup.
A concerto is a classical musical composition that highlights a solo instrument against a full orchestra background. Bach is a composer known for writing concertos. A piano, violin, flute or other instrument plays solo parts in a concerto that are supported or highlighted by an orchestra.
