The correct answer is: A. Air pressure.
The Earth’s atmosphere is divided into layers based on the vertical profile of air pressure. The lowest layer, the troposphere, extends from the surface of the Earth to an average height of about 12 kilometers (7.5 miles). The troposphere is where most of the Earth’s weather occurs. The next layer, the stratosphere, extends from the top of the troposphere to an average height of about 50 kilometers (31 miles). The stratosphere is where the ozone layer is located. The next layer, the mesosphere, extends from the top of the stratosphere to an average height of about 85 kilometers (53 miles). The mesosphere is the coldest layer of the atmosphere. The next layer, the thermosphere, extends from the top of the mesosphere to an average height of about 600 kilometers (373 miles). The thermosphere is the hottest layer of the atmosphere. The next layer, the exosphere, extends from the top of the thermosphere to an average height of about 10,000 kilometers (6,200 miles). The exosphere is the outermost layer of the atmosphere and is where the atmosphere merges with space.
Air pressure is the force exerted by the weight of the air above a given point. The higher the air pressure, the more air is above that point. Air pressure decreases with altitude because there is less air above a given point at higher altitudes.
Air temperature is the average kinetic energy of the molecules in a given volume of air. The higher the air temperature, the faster the molecules are moving. Air temperature decreases with altitude because there is less air to absorb the heat from the sun at higher altitudes.
Air density is the mass of air per unit volume. The higher the air density, the more air is in a given volume. Air density decreases with altitude because there is less air at higher altitudes.
Wind speed is the rate at which air is moving. Wind speed is not a factor in the division of the Earth’s atmosphere into layers.