The part of the brain responsible for reasoning and problem-solving is the prefrontal cortex, located in the frontal lobe. This region is crucial for higher cognitive functions, including decision-making, planning, and social behavior. It integrates information from various brain areas and plays a significant role in personality development and self-control. Damage to the prefrontal cortex can lead to difficulties in reasoning and emotional regulation, demonstrating its essential role in human cognition and behavior.