What Should a Psychologist Test to Determine Whether a Given

What should a psychologist test to determine whether a given child is in the concrete-operations stage or the formal-operations stage?

A) whether the child reaches around a barrier to retrieve a hidden toy
B) whether the child understands that an object maintains its weight and volume after its shape changes
C) whether the child can answer abstract and hypothetical questions
D) whether the child can distinguish between appearance and reality