Which gas constitutes only a small percentage of the earth's atmosphere, ranging from trace amounts to 4% by volume?

Water vapor is the correct answer because it is known to vary significantly in concentration within the Earth's atmosphere, typically ranging from trace amounts up to approximately 4% by volume depending on the temperature and humidity of the air. Unlike other gases such as oxygen and nitrogen, which are more stable in their concentrations (approximately 21% and 78%, respectively), water vapor is in a constant state of flux due to processes like evaporation and condensation. Its varying presence plays a critical role in weather and climate, as it is a key greenhouse gas and contributes to cloud formation and precipitation. This variability is a distinctive characteristic that differentiates water vapor from the more consistently prevalent gases in the atmosphere.