Final answer:
Carbon dioxide is the gas in the atmosphere used by plants to form sugars, is soluble in water, and makes rain naturally acidic.
Step-by-step explanation:
The gas in the atmosphere used by plants to form sugars, is soluble in water, and makes rain naturally acidic is Carbon Dioxide (C). It is naturally present in the atmosphere because most organisms produce it as a waste product of metabolism. Carbon dioxide dissolves in rainwater and reacts with it to form carbonic acid, which contributes to the acidity of rain. This process is a part of the carbon cycle and is essential for photosynthesis, the process by which plants convert carbon dioxide and sunlight into sugars.