The correct answer is D. Air moving along lines of constant longitude
Explanation:
In geography, the word meridian refers to an imaginary line that connects the North pole and the South pole, this includes the main meridian called Prime Meridian as well as multiple longitude lines that determine East and West.
On the other hand, the meridional circulation refers to a natural phenomenon in which air and air masses move from North to South and vice-versa following longitude lines or the imaginary lines that divide the planet vertically. Additionally, the meridional circulation affects climate especially in terms of extreme weather. According to this, the meridional circulation means "Air moving along lines of constant longitude".
