Final answer:
Franz Joseph Haydn's positive and well-balanced character is most reflected in his Symphonies (A), a form he greatly contributed to during the Classical era.
Step-by-step explanation:
The question asks which form of musical composition best reflects Franz Joseph Haydn's positive and well-balanced character. The answer is A. Symphonies. Haydn is often referred to as the 'Father of the Symphony' and the 'Father of the String Quartet' because of his significant contributions to these forms. His work in the symphonic form particularly reflected the balance and clarity associated with the Classical era, and his symphonies often express a positive and vibrant character. While Haydn also composed in other forms, such as operas, sonatas, and concertos, his symphonies best encapsulate the qualities mentioned in the question. Much like his contemporaries Mozart and Beethoven, Haydn's works adhered to the structural and stylistic norms of the time, yet his symphonies in particular were imbued with a unique jovial and balanced spirit that matched his own disposition.