What Does the Term Horizontal Décalage Refer to with Respect

What does the term horizontal décalage refer to with respect to the acquisition of complex mental concepts?

A) They cannot be acquired more quickly, no matter what type of training is provided.
B) They can be acquired more quickly by children if appropriate verbal cues are used.
C) They are usually acquired in a single step or stage.
D) They are not acquired in a single step and may show uneven developmental progress.