Final answer:
The respiratory gas found dissolved in rainwater is carbon dioxide, as air consists mostly of nitrogen, oxygen, and smaller amounts of carbon dioxide and other gases.
Step-by-step explanation:
The respiratory gas that is found dissolved in rainwater from the atmosphere is carbon dioxide (b). Air is a mixture of gases consisting primarily of nitrogen (78.6%), oxygen (20.9%), and in smaller amounts, other gases such as carbon dioxide (0.04%). While nitrogen is the most abundant and can be dissolved in water, it is relatively inert and does not easily form compounds with other elements. Carbon dioxide, though present in minor amounts in the atmosphere, dissolves in rainwater and forms a weak acid (carbonic acid), which is a process integral to various biological and chemical cycles. Oxygen is also found dissolved in rainwater, but it is not the answer to the question asked. Hydrogen is virtually absent in the atmosphere due to its light molecular weight and the fact that it escapes into space over time.