Which invention is associated with Eli Whitney?

Eli Whitney is most famously associated with the invention of the cotton gin, which revolutionized the cotton industry in the United States. Patented in 1794, the cotton gin mechanized the process of separating cotton fibers from seeds, significantly increasing cotton production efficiency. This invention not only transformed agriculture in the southern United States by making cotton a highly profitable crop but also had profound socio-economic implications, contributing to the expansion of slavery in the South as demand for cotton grew.

The other inventions mentioned, such as the telegraph, McCormick's reaper, and the steam engine, while impactful in their own right, were developed by different inventors and do not relate to Whitney's contributions. The telegraph was notably developed by Samuel Morse, the reaper was invented by Cyrus McCormick, and the steam engine saw significant advancements by figures like James Watt. Hence, Whitney's cotton gin remains the key association with his legacy.