The correct answer is B. Cool air is sinking.
Explanation
The image shows the behavior of the wind when subjected to the rays of the sun and when it is under a cloud. On the left, the clouds are shown and below them, the downward-pointing arrows that represent the descending air. On the left side there are arrows pointing upwards and a warm environment from the sun's rays. This image is a sample of the convection of the air by temperature, that is to say that when the air has a hot temperature, the air tends to rise, while when the air has a cold temperature, it tends to decrease. Therefore, the arrows in the circle indicate that Cool air is sinking. So, the correct answer is option B.