Which event was most crucial in steam becoming the leading energy source in the united states by the year 1900?

The discovery an opening of large coal deposits.

Explanation:

Coal is the most widely available fossil fuel in the world and one of the first sources of energy used by man. It was widely used in the period of the Industrial Revolution in Europe. In the United States, several coal deposits were found in the mid-1900s. These deposits influenced greatly the United States industry and were the crucial factor in making steam the main source of energy in the United States in the year 1900.