In the Miller-Urey experiment (1953), a mixture of gases that was thought to represent the early Earth's atmosphere was subjected to electrical discharges to simulate lightning. The gas mixture used was composed of hydrogen (H\(_2\)), methane (CH\(_4\)), and ammonia (NH\(_3\)) in a 2:1:1 ratio, which produced amino acids and other organic compounds.
Thus, the correct answer is 2:1:1 (H\(_2\): NH\(_3\): CH\(_4\)).