Which Scenario Describes a Child Who Has Mastered the Concept

Which scenario describes a child who has mastered the concept of conservation of mass?

A) Trent knows that objects still exist, even when he cannot see them.
B) Keisha understands that she can get more milk if she finishes the milk she has.
C) Jeremy remembers that he ate all of the crackers that his father put in his lunch box.
D) Jane knows that she has the same amount of clay in her round ball as Rico has in his flat disk.