Answer:
The correct answer is D. Formal operational stage.
Step-by-step explanation:
According to Piaget, one has reached this final stage when he/she is able to think in an abstract way. This means that one can think about things that are not physically there, so the person has to imagine them.
In this case, the person asking about the family would have to see the siblings physically in order to know who is the youngest. Someone who has reached the formal operational stage would be able to deduce who is the youngest by picturing them in his/her mind.