Final answer:
Emperor Joseph II of the Holy Roman Empire commissioned music from Mozart. This era saw the transition from classical to Romantic styles, influencing future composers.
Step-by-step explanation:
The emperor who commissioned music from Wolfgang Amadeus Mozart was Emperor Joseph II of the Holy Roman Empire. Mozart, a significant figure in classical music, composed a variety of works, including operas, symphonies, and chamber music, under the patronage of Emperor Joseph II during the late 18th century. This was a time when many other composers like Beethoven started their careers in classical music and later transitioned into the Romantic era, producing compositions that were stylistically advanced for the time, challenging their audiences, and paving the way for future composers like Brahms, Tchaikovsky, and Wagner.