What type of air mass produces stable air associated with smooth air, poor visibility, and stratiform clouds?

The correct choice describes a warm air mass moving over a cold surface. This scenario leads to the development of stable air conditions. When warm, moist air moves over a cooler surface, it cools down as it comes in contact with the colder air underneath. This cooling process can promote stability in the atmosphere, as the warmer air becomes denser and stays nearer to the ground rather than rising.

Stable air is characterized by smooth conditions, which typically result in a lack of turbulence—ideal for comfortable flying. Additionally, this type of airflow can lead to poor visibility due to the formation of stratiform clouds, which are layered and uniform, often resulting from the gradual cooling of the air mass. Consequently, these conditions hinder the mixing of air and can lead to situations such as fog, contributing further to reduced visibility.

Other types of air masses do not generally result in the same stable conditions. For instance, a warm air mass by itself may not produce stable conditions if it interacts with other factors or air masses, and cold air masses moving over warm surfaces tend to create instability, promoting turbulence and vertical air movement.