The correct answer is B) warm moist air being blown over a colder surface. Advection fog is caused by the movement of warm, moist air over a colder surface, such as a body of water or land. As the warm air moves over the colder surface, it cools and the moisture in the air condenses, forming a dense fog. This process of air movement and cooling is known as advection, which is why this type of fog is called "advection fog." The other options are incorrect because they do not directly describe the primary cause of advection fog. Option A, saturation of cold air by rain, is more associated with precipitation fog. Option C, air being warmed above the dew point, is the opposite of what happens in advection fog formation. Option D, rapid cooling of air near the surface, is more characteristic of radiation fog, which forms at night due to surface cooling.