Explanation:

The atmosphere is divided into several layers based on temperature changes with altitude. The layers are as follows:

1. Troposphere: This is the layer closest to the Earth's surface, extending from the surface up to an average altitude of about 12 km at the poles and 18 km at the equator. The troposphere contains approximately 75% of the total atmospheric mass and almost all weather phenomena occur in this layer. The composition of gases in the troposphere includes nitrogen, oxygen, argon, carbon dioxide, water vapor, and trace amounts of other gases, but the percentage of helium is relatively low.

2. Stratosphere: The stratosphere is located above the troposphere and extends up to an altitude of about 50 km. This layer contains the ozone layer, which absorbs most of the Sun's harmful ultraviolet (UV) radiation. The composition of gases in the stratosphere is similar to the troposphere, with relatively low concentrations of helium.

3. Exosphere: The exosphere is the outermost layer of the atmosphere, extending from about 500 km above the Earth's surface. It gradually merges with the vacuum of space. The exosphere is composed of extremely low-density gases, including helium and hydrogen. Since helium is a light gas, it tends to rise to the upper layers of the atmosphere, making the percentage composition of helium gas maximum in the exosphere.

4. Ionosphere: The ionosphere is a region within the Earth's upper atmosphere, extending from about 60 km to 1,000 km above the surface. It is characterized by the presence of ions and free electrons, which are formed due to the ionization of atmospheric gases by solar radiation. The ionosphere is also known for its ability to reflect radio waves, allowing long-distance radio communication. While the ionosphere contains ionized helium, the overall percentage composition of helium in this layer is not as high as in the exosphere.

Therefore, the percentage composition of helium gas is maximum in the exosphere (option C).