Final answer:
Niccolò Paganini was the 19th-century Italian violinist who designed his 'Napoleon sonata' to be played on just one string.
Step-by-step explanation:
The 19th-century Italian violinist who designed his 'Napoleon sonata' to be played on just one string was Niccolò Paganini. Paganini was known for his virtuosic violin skills and innovative compositions, often pushing the boundaries of what was considered acceptable at the time. His ability to play complex and challenging pieces using only one string showcased his remarkable technical prowess.