2. Air
Explanation:
Exosphere The outermost layer of the earth's atmosphere is known as the exosphere which lies between 400 and 1000 kilometers. At such a great height the density of atoms in the atmosphere is extremely low. Hydrogen and helium gases predominate in this outermost region. At the outermost boundary of our atmosphere, the kinetic temperature may reach a fantastically high value of about 5568 degree Celsius. However, this temperature in its real sense is entirely different from the temperature at the earth's surface.