Final answer:
The appearance of a plane flying slowly at a high altitude is explained by the monocular depth cue known as relative size, which is how we perceive the depth of objects based on their actual size versus their apparent size. Therefore the correct answer is Relative size.
Step-by-step explanation:
When observing a plane flying high in the sky and it appears to move slowly, despite the fact that it is traveling at a high velocity, this phenomenon is explained by the monocular depth cue known as relative size.
Relative size is a cue that allows us to perceive the depth of objects based on their known size versus how large they appear in our field of vision.
In this case, the plane appears small compared to its actual size, which makes it appear as though it's moving slowly, although it is actually moving quickly at a great distance from the observer.
This is in contrast to binocular depth cues such as binocular disparity, which rely on both eyes to perceive depth. Monocular cues can be used to create illusions of depth such as in 3-D movies, where disparate information is integrated to give the illusion of three dimensions.