जीन पियाजे के संज्ञानात्मक विकास के सिद्धांत के अनुसार, प्रभावी अधिगम (learning) मुख्य रूप से किसके माध्यम से प्राप्त होता है?

A child has developed the ability to conserve and arrange items serially. Their thought processes are rational and directed to concrete, real events, but they struggle with highly abstract or hypothetical problems. At which stage of Piaget's cognitive development is this child?

Assertion (A): A twelve-year-old child is unable to perform tasks involving conservation. Reason (R): Children in the formal operational stage often find it challenging to take into account multiple aspects of a situation. Choose the correct option based on Jean Piaget's theory of cognitive development: