What is the main composition of clean dry air?

The main composition of clean dry air consists of approximately 78% nitrogen and 21% oxygen, making option B the correct answer. Nitrogen is the most abundant gas in the atmosphere, playing a crucial role in maintaining the stability and balance of atmospheric composition. Oxygen, while essential for the survival of most life forms on Earth, is present in a smaller proportion. This balance of gases is vital for various processes, such as respiration in living organisms and maintaining the ecosystem's health.

The other options reflect incorrect compositions, as clean dry air does not contain such high levels of oxygen or equal proportions of nitrogen and oxygen.