In a Baroque concerto, a refrain played by the whole orchestra is called a ritornello, which serves as a recurring theme. This structural element is crucial in the musical form of Baroque concertos. Other terms mentioned have different meanings and applications in music. ;
In a Baroque concerto, the part played by the whole orchestra known as a refrain is called a ritornello. This term signifies a recurring theme that contrasts with solo passages, forming a key element of the concerto's structure. Therefore, the answer is D. A ritornello.
;