Final answer:
Joseph Haydn was a prolific composer known for writing 68 string quartets and 104 symphonies, with the latter being the answer to the student's question.
Step-by-step explanation:
Joseph Haydn, a renowned composer, demonstrated his productivity in the realm of classical music with his extensive catalogue, which includes 68 string quartets and 104 symphonies. The answer to the student's question is D - symphonies. Haydn's creative output was a testament to his mastery in various classical forms which continued to be embraced and emulated by Romantic era composers, who sought to progress these traditions while respecting the established rules of rhythm, melody, harmony, and performance practice handed down from the Classical period.