Final answer:
The conditions that would lead to the most vibrant colors in a sunset are a clear sky and pollution-free air.
Step-by-step explanation:
The conditions that would lead to the most vibrant colors in a sunset are a) Clear sky and pollution-free air.
When the sky is clear and there is no pollution, the sunlight can pass through the atmosphere without much interference. As the sun approaches the horizon, its light has to pass through a larger portion of the Earth's atmosphere, resulting in increased scattering of blue and green light. This scattering causes the remaining sunlight to be richer in red and orange colors, creating a more vibrant sunset.