Final answer:
Mr. Otis' great invention was the cotton gin, which revolutionized the cotton industry in the early nineteenth century.
Step-by-step explanation:
Mr. Otis' great invention was the cotton gin, which was invented by Eli Whitney and patented in 1794. The cotton gin was a device that sped up the production of cotton by efficiently removing the seeds from the raw cotton. This invention revolutionized the cotton industry and had a significant impact on the economy and social fabric of the early nineteenth century.