Which of the following works by Wolfgang Amadeus Mozart remained unfinished at the time of his death?

"Requiem Mass in D Minor", K 626, is a work by Wolfgang Amadeus Mozart, left incomplete at his death on December 5, 1791. Until the late 20th century, the work was most often heard as it had been completed by Mozart’s student Franz Xaver Süssmayr. Later completions have since been offered, and the most favorably received among these is one by American musicologist Robert D. Levin.

According to a contract that Mozart signed and an attorney witnessed, the Requiem was commissioned by the Count Franz von Walsegg-Stuppach. The count pretended to possess some musical ability and liked to pass off the work of others as his own. The new requiem, intended as a tribute to the count’s wife, was part of that game. Therefore, he insisted that Mozart was neither to make copies of the score nor to reveal his involvement in it, and that the first performance was reserved for the man who commissioned the piece.

Mozart suffered from failing health towards the end of his life, and he worked on the "Requiem Mass in D Minor" when strength permitted. Several friends came to his apartment on December 4, 1791 to sing through the score-in-progress. Yet his condition worsened, and, by the time of Mozart’s death early the next morning, he had finished only the Introit. The Kyrie, Sequence, and Offertorium were sketched out. The last three movements—Benedictus, Agnus Dei, and Communio—remained unwritten, and nearly all the orchestration was incomplete.