Marcus looks at the magnolia trees outside his bedroom window. He can distinctly see the individual leaves and flowers on the nearest trees but not on the more distant trees. This example BEST illustrates the depth cue of:

Texture gradient

Explanation:

In psychology, Texture gradient is said to be the distortion in size which closer objects have compared to objects farther away.

That is to say that, objects that are closer to you appear distorted when compared to when the same objects are farther away.

It also involves groups of objects that appear to be denser as they move farther away.

Noticing a certain amount of detail based on how close something is, and thus having a sense of depth perception is referred to as Texture Gradient.

Texture Gradient is somehow related to relative size but it is a depth cue in its own right.

According to Gibson in 1950 most surfaces, such as walls, roads, flowers and field of flowers in bloom, have a texture and as the surface gets farther away from us this texture gets finer and appears smoother. This is an illustration of gradient texture depth cue and it mirrors how Marcus saw the Magnolia tree and it's leaves.