The United States of America was a major source of raw cotton for Britain until 1861 primarily due to the cotton industry's reliance on cotton from the American South, known as the "Cotton Kingdom." American cotton, particularly long-staple cotton, was highly sought after by British textile manufacturers during the Industrial Revolution because of its quality and suitability for mechanized cotton spinning and weaving. The American South, with its warm climate and fertile soil, became a major cotton-producing region, and the cotton produced there was a significant driver of the global cotton trade during this period. The outbreak of the American Civil War in 1861 disrupted this trade, leading Britain to look for alternative sources of cotton.

So, the correct option is [1].