Final answer:
Mozart was composing the 'Requiem' at the time of his death in 1791, during the Classical period, which is a specific era of Western classical music.
Step-by-step explanation:
Wolfgang Amadeus Mozart was composing the Requiem when he died in 1791. The Requiem is a mass in D minor for the dead, one of the most famous pieces of choral music and a significant work within the Western classical tradition. Mozart composed during the Classical period, which should not be confused with general "Classical" music but refers specifically to the second half of the eighteenth century, a time when Mozart and Haydn were the most renowned composers. The Requiem was left unfinished at Mozart's death and was later completed by one of his students, Franz Xaver Süssmayr. Despite the romanticized stories around its composition, the Requiem remains one of Mozart's most profound works, evoking both the beauty and solemnity of life's final passage.