The atmosphere of the Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth's gravity. The atmosphere of Earth protects life on Earth by creating pressure allowing for liquid water to exist on the Earth's surface, absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night.

The atmosphere consists of roughly 78% Nitrogen, 21% Oxygen, and 1% other gases. Of those other gases, water vapor is the most abundant and accounts for 0.25-4% of the atmosphere. Carbon dioxide (CO2) is the second most abundant gas in the atmosphere and accounts for roughly 400 parts per million (ppm).

97% of the atmosphere lies within the first 5 km (3.1 miles) of the surface of the Earth. This layer is known as the troposphere, and it is the layer where the majority of weather occurs. The troposphere is the most dense layer of the atmosphere and contains roughly 75% of the atmosphere's mass. The temperature of the troposphere generally decreases with altitude, which is why airplanes fly at high altitudes to avoid the turbulence associated with air pockets of cooler air.

Beyond the troposphere is the stratosphere, which extends from 5 km (3.1 miles) to 50 km (31 miles) above the Earth's surface. The stratosphere contains the ozone layer, which absorbs much of the incoming ultraviolet radiation from the sun and thus protects life on Earth. The temperature of the stratosphere generally increases with altitude, which is why high flying aircraft such as commercial airliners and military jets fly in this layer.

Therefore, the correct answer is Option C 5 km.

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