Question

which phrase best describes the tropopause?
the boundary between the troposphere and the stratospherenthe boundary between the troposphere and the mesospherenthe boundary between the troposphere and the ionospherenthe boundary between the troposphere and the exosphere

Explanation:

The troposphere is the lowest layer of Earth's atmosphere. Above it is the stratosphere. The boundary between the troposphere and stratosphere is called the tropopause. The mesosphere is above the stratosphere, the ionosphere is part of the thermosphere (which is above the mesosphere), and the exosphere is the outermost layer of the atmosphere.

Answer:

the boundary between the troposphere and the stratosphere