For how long has the proportions of the different gases in the atmosphere remained much the same?

The proportions of the different gases in the atmosphere have remained relatively stable for approximately 200 million years. This stability is significant because it indicates that, despite various geological and biological changes on Earth, the composition of the atmosphere has not varied dramatically. This consistency is essential for maintaining life as we know it, as the balance of gases like nitrogen, oxygen, and carbon dioxide plays a crucial role in processes such as respiration and photosynthesis.

The answer of 200 million years highlights a timescale that encompasses the period during which major events, such as the rise of mammals and the development of modern ecosystems, occurred alongside the atmospheric stability. While the other choices might represent significant geological periods, they either extend too far back in time or do not accurately reflect the timeframe during which we observe the atmosphere's composition as it is today.