B because the higher the temperature of air, the faster the molecules move, as the air is heated up, the air molecules move faster, which when the temperature is higher, the air molecules are less dense
Density is defined as the amount of mass per unit volume. Now, according to Boyle's law that says that at a constant temperature, the volume occupied by a gas is inversely proportional to the pressure. If we apply heat we can deduce that the expansion of a gas is a function of temperature (Gay-Lussac Law). According to this law, if the same mass of air occupies a larger volume, its density will be lower. In this way, the density of the air is inversely proportional to the temperature. That is, at higher temperatures, lower density and at lower temperatures, higher density.
