Final answer:
The tropopause is the boundary between the troposphere and the stratosphere; it is defined as Option 2. It marks the transition from the zone where temperature decreases with altitude to the zone where it increases due to UV absorption by ozone.
Step-by-step explanation:
The tropopause can be defined as the boundary between the troposphere and the stratosphere. This boundary marks the end of the troposphere, the lowest layer of Earth's atmosphere, which extends from the planet's surface to an altitude of about 11-13 km (7-8 miles) where the temperature decreases with altitude. Beyond the tropopause lies the stratosphere, where the temperature starts to increase again due to the absorption of ultraviolet radiation by ozone, ultimately leading to the ozone layer.
The correct definition of the tropopause is Option 2: The boundary between the troposphere and the stratosphere, distinguishing it from other atmospheric boundaries such as that between the troposphere and the ionosphere, mesosphere, or exosphere. The tropopause is also traditionally the coldest part of the atmosphere.