Final answer:
Art song was most notably popular in Romantic music of the 19th century, with composers such as Schubert and Brahms making significant contributions to the form.
Step-by-step explanation:
The art song became a popular repertory in Romantic music of the 19th century (Option D). This time was marked by a profound transformation in the arts cumulating in a variety of new styles of performance and forms of expression. During this period, the industrial revolution brought increased literacy, leisure, and an emerging middle class that furthered the demand for music. Composers of the Romantic era like Schubert, Schumann, Brahms, and others, elevated the art song to a prominent position in Western music.