Fronts are defined as:

• A. A layer of the atmosphere that separates the troposphere from the stratosphere.
• B. The transition zone between two different air masses at the Earth's surface.
• C. A region with no weather changes.
• D. The boundary between two air masses in the upper atmosphere.

Answer: (B)

Fronts are boundaries where air masses with different properties meet and interact. This boundary marks a transition in temperature, humidity, and wind, and the air masses don’t mix easily, often causing lifting and cloud formation as one mass overrides or slides under the other. The best description among the options is that it’s the transition zone between two different air masses at the Earth's surface, because that captures the idea of two distinct air parcels meeting and creating noticeable weather changes.

The other descriptions don’t fit as well. The layer that separates the troposphere from the stratosphere is the tropopause, not a front. A region with no weather changes isn’t what fronts produce—fronts are typically associated with shifts in weather, such as clouds and precipitation. A boundary limited to the upper atmosphere ignores the surface interaction that defines how fronts are observed and tracked in everyday weather.