Inversion of temperature means

In the troposphere, air temperature normally DECREASES with increasing altitude (the environmental lapse rate, roughly 6.5°C per km) because the atmosphere is heated chiefly from below by the Earth's surface. A temperature inversion is the reversal of this normal pattern: a layer in which temperature INCREASES with height instead of decreasing, so a warmer layer of air sits above a cooler layer near the surface.

Applying this definition to the given options: only the profile in which temperature rises as height increases matches what "inversion" denotes.

• Constant temperature with height describes an isothermal layer — temperature stays flat, so there is no reversal at all, and this does not fit the term.

• Fluctuating temperature with height describes an irregular, up-and-down profile, not the smooth, sustained reversal that defines an inversion.

• Decrease of temperature with height describes the ordinary, non-inverted atmospheric condition (the usual cooling with altitude) — the standard state that inversion is defined as a departure from, not the departure itself.

Cross-check with familiar real examples: a clear, calm winter night produces a radiative inversion near the ground, and the base of the stratosphere is another layer where temperature rises with altitude — in both cases air temperature increases as height increases, confirming this is the profile the term "inversion" refers to.