Answer: Option D; boundary where temperature trends reverse in the atmosphere.
Explanation: Tropopause is the boundary in the Earth's atmosphere between the troposphere and the stratosphere, and is a thermodynamic gradient stratification layer.
Stratopause is the boundary between the stratosphere and the mesosphere, here you will find the highest temperature in the atmosphere
Mesopause is the point of minimum temperature at the boundary between the mesosphere and the thermosphere atmospheric regions.
So all of them are boundaries, and in each you will see that in all of them a change in temperature occurs, so the correct option will be D.