Before the invention of the cotton gin, cotton was notoriously difficult to profitably produce. Cotton is encased in a hard shell - extracting it and removing excess material was a time consuming and costly process. The cotton gin, by mechanizing the process, dramatically increased the amount of cotton that a plantation owner could produce and made the cash crop extremely profitable again. However, in order to process the cotton it first needed to be picked - which called for an influx of a massive number of forced laborers (slaves). More than one of the answers presented here are true, however the key component is "most significant". The massive importation of slaves to harvest cotton probably prolonged the death of slavery by several years in the South and has ramifications for the economic, social and political make-up of America to this day. 

It was the establishment of the telegraph that rendered the Pony Express obsolete. 

Gold was found at Sutter’s Mill in Northern California in January of 1848, weeks before the Treaty of Guadalupe Hidalgo made California an American territory.  The ever present desire for gold led people to endure brutal travel either overland in stagecoaches or wagon trains, or by sailing to Panama and crossing the Isthmus by foot.

Samuel Colt, of Colt 45 fame, is the inventor credited with mass-producing the revolver for the first time.

The Panic of 1873 was one of the worst world wide economic depressions until the Great Depression occurred in 1929. The Panic began after widespread railroad speculation in the post Civil War era overstretched financial resources throughout the country. After the failure of a handful of railroads, banks themselves failed causing widespread unemployment, inflation, and bank runs. The depression would last until 1879, covering all of Ulysses S. Grant's second term in office.

The cotton gin (short for engine) was invented by the Connecticut born Eli Whitney in 1793, and quickly the model spread throughout Southern plantations. The cotton gin was able to separate the cotton fibers from the seeds extremely quickly and efficiently, and allowed the production of cotton to increase exponentially. This meant that Southern plantation owners also increased their importation and trade of slaves, leading to a boom in the Southern slave population.

A rich and powerful businessman of the 1800s, pejoratively named a Robber Baron, would not have supported the workers' right to unionize as this would have worked against his business interests.

The Whiskey Rebellion was an oftenviolent protest by grain and corn farmers against Federal taxes levied on whiskey intended, primarily, to help pay for the nation's assumed war debt.

The Homestead Acts, first passed during the Civil War, offered free land grants to anyone who had not previously “taken up arms against the Union.” The land offered generally, although not always, amounted to roughly one-hundred and sixty acres. Between the period of 1862 and 1934, the Federal government granted 1.6 million homesteads to American families (roughly ten-percent of United States land was appropriated) and it was a big motivational factor behind the consistent westward expansion of the American people in that time period. The Homestead Acts were officially repealed in the 1970s. 

The success of Bacon's Rebellion, in 1676, allowed for the change from indentured servants to slaves as the first major migration of indentured servants became free. The rebellion looted plantations and took over the government for a short period of time, but later led to the increase of African slaves.