Final answer:
The stratosphere is the atmospheric zone where the temperature remains fairly constant, due to the absorption of UV radiation by the ozone layer.
Step-by-step explanation:
The atmospheric zone where the temperature remains fairly constant is the stratosphere. This is layer B from the given options. The troposphere is the lowest part of the atmosphere, extending from Earth's surface to about 11-13 km (7-8 miles), where the temperature decreases with altitude. In contrast, the stratosphere extends from above the troposphere to about 50 kilometers above the surface and features a relatively stable temperature, largely due to the absorption of ultraviolet (UV) radiation by the ozone layer. The ionosphere is part of the upper atmosphere, from about 60 kilometers above the surface into the thermosphere, and is characterized by ionization due to solar radiation, not temperature stability.