The correct answer is option 3: Stratosphere.

In the troposphere, which is the layer closest to the Earth`s surface, temperature generally decreases with increasing altitude. This is because the troposphere is primarily heated by the Earth`s surface.

In the ionosphere, temperature trends become more complex due to the presence of ions and charged particles. Temperature variations in the ionosphere are not solely dependent on altitude.

The stratosphere, on the other hand, is located above the troposphere and contains the ozone layer. In the stratosphere, the temperature remains relatively constant with increasing altitude or even slightly increases. This is due to the absorption of solar radiation by the ozone layer, which results in a temperature inversion.

The mesosphere, located above the stratosphere, is the layer where temperature decreases with increasing altitude.