Answer:
Mental representation.
Step-by-step explanation:
Sensorimotor Stage (Birth-2 yrs)
The main achievement during this stage is Object Permanence - knowing that an object still exists, even if it is hidden (the ball under the chair).
It requires the ability to form a mental representation (i.e., a schema) of the object.
Baby Luke has the schema of everything that happened: the ball bounces out of the playpen and rolls under the chair. He underestands that the ball is there (where he last saw it) even if it's hidden.