Final answer:
The highest concentration of atmospheric gases near the Earth's surface is due to Earth's gravity which pulls gases closer, creating a denser atmosphere.
Step-by-step explanation:
The primary reason for the highest concentration of atmospheric gases near the Earth's surface is due to Earth's gravity. Out of the given options A through D, gravity is the fundamental force that causes the gases to be pulled closer to the Earth's surface, thus creating a denser layer of air. Earth's magnetic field, moon cycles, and internal radioactive processes have other effects but do not explain this concentration of gases.
When discussing the atmosphere and its gases, it can also be relevant to consider the greenhouse effect. This is a natural process wherein certain greenhouse gases, like carbon dioxide (CO2), absorb infrared radiation that is reemitted by the Earth's surface and then radiate it back, warming the planet. Without this effect, the sun's energy would simply be radiated back into space, and Earth would be much colder.