Final answer:
Air drainage into mountain valleys, radiative cooling of the ground at night, and a lowering of the stratosphere during periods of low Sun activity can all lead to a temperature inversion.
Step-by-step explanation:
A temperature inversion occurs when warm air is held above cooler air, resulting in a deviation from the normal temperature profile with altitude. It almost always refers to an inversion of the thermal lapse rate. In this case, all of the given options can lead to a temperature inversion. Air drainage into mountain valleys can create a stable layer of warm air above cooler air in the valley. Radiative cooling of the ground at night can result in a layer of cooler air near the surface, with warmer air above. Lowering of the stratosphere during periods of low Sun activity can also lead to a temperature inversion.