Elena perceives the hills in Leonardo da Vinci's painting as being in the background because they are painted somewhat blurry and with a bluish tint.Leonardo da Vinci made use of the monocular depth cue:

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The bluish tint and blurry appearance of the hills indicate the use of haze, which is a monocular depth cue that involves objects appearing less sharp or distinct as they get further away due to the atmosphere causing a decrease in contrast and color saturation.

Elena Perceives the Hills in Leonardo Da Vinci's Painting as Being