Turbulence in flight is classified into four levels: light, moderate, severe and extreme. Only moderate, severe and extreme turbulence are considered operationally significant and require an explicit forecast (in SIGMET, AIRMET or TAF / SIGWX charts) because they affect the comfort and the safety of the flight. 

Light turbulence, on the other hand, is omnipresent in the lower atmosphere during daytime: heating of the ground produces convective thermals, mechanical turbulence develops over irregular terrain, and wind shear creates small scale eddies near the surface. This kind of turbulence is so common that it is not specifically forecast, and the crew has to assume it is always present whenever flying close to the ground or in the daytime convective layer.

Severe and moderate CAT, by contrast, are only encountered in specific conditions (jet stream proximity, mountain waves, frontal shear) and are explicitly forecast when detected. 

The level of turbulence that the crew must always expect, regardless of any explicit forecast, is therefore light turbulence (thermal).

Moderate clear air turbulence (CAT) → INCORRECT. Moderate CAT is a significant phenomenon that is explicitly forecast in SIGMETs and on SIGWX charts when expected. 

Light turbulence (also called thermal turbulence) → CORRECT. Light turbulence is so common in the daytime convective layer and over irregular terrain that it is never specifically forecast; the crew must always assume that some level of light turbulence may be encountered, particularly when flying at low altitude during the day. 

Moderate and severe turbulence → INCORRECT. Moderate and severe turbulence are considered operationally significant and are always forecast when expected, in SIGMETs, AIRMETs or significant weather charts. They are not present everywhere by default and would require specific atmospheric conditions to occur.

Heavy turbulence below cumulus clouds → INCORRECT. Turbulence under cumulus clouds depends on the depth and the intensity of the convective activity. While moderate or severe turbulence can occur under well developed cumulus, this is not the baseline turbulence to be expected always; it is reported when significant convective activity is forecast.