Final answer:
Differences in density keep two unlike air masses separate, often caused by temperature differences influencing how rapidly particles move in hot versus cold air and resulting in pressure differences. Therefore, the correct option is c.
Step-by-step explanation:
When two unlike air masses meet, what usually keeps them separate is c. differences in density. This density difference is primarily because hot air is less dense than cold air, which results from the fact that the particles in hot air are moving more rapidly. Density differences also create pressure differences, which can further maintain the separation of air masses. For instance, the pressure difference between two objects causes heat transfer. In terms of moisture content, warm air can hold more moisture than cold air, and this influences precipitation patterns. Increasing humidity may lead to altered evaporation rates due to the change in the water vapor pressure gradient.