Final answer:
Franz Xaver Süssmayer completed the unfinished portions of Mozart's Requiem.
Step-by-step explanation:
The correct answer is a. Franz Xaver Süssmayer. He was a composer and conductor who completed the unfinished portions of Mozart's Requiem after Mozart's death. Süssmayer was a student and close friend of Mozart, and he was familiar with his composition style, making him a suitable choice to finish the work.