Answer:
C. Object permanence
Step-by-step explanation:
This question should be answered looking at the definition of all four phrases:
A. conservation refers not using resources in a wasteful manner.
B. Sensorimotor Memory refers to your muscles and sensorimotor system developing a memory to do certain activities over time.
C. Object permannence refers to the understanding that objects don't disappear just because you cannot see them.
D. Egocentrism is the inability to see something through someone else's perspective.
From looking at the definitions it is clear that the answer is C, since Ellie forgot about the truck as soon as she could no longer see it.