• Fore-brain is main thinking part of human brain.
• Fore-brain comprises of Cerebrum, Thalamus and Hypothalamus.
• The cerebrum controls all voluntary activities like thinking, learning, reasoning, etc.
• Hind-brain comprises of Pons, Cerebellum and Medulla oblongata. The Medulla contains centres of respiration, cardiovascular reflexes, etc.
• Cerebellum maintains orientation, posture and equilibrium of body.
• Mid-brain comprises of four lobes called corpora quadrigemina. 

The correct answer is Cerebrum.

Explanation:

• The main thinking part of human brain is cerebrum.
• The cerebrum is  large outer part of  brain
• It controls reading, thinking, learning, speech, emotions, and planned muscle movements like walking.

• The cerebrum (a major part of forebrain) is main thinking part of brain. It has sensory, motor, and association areas.

Additional Information

• The human brain is divided into three parts or compartments with different functions: Fore-brain, Midbrain, and hindbrain.
• The human brain with spinal cord makes  central nervous system.
• Fore-brain is main thinking part and controls voluntary actions.
  • It processes sensory information which is collected from various organs of body such as ears, eyes, nose tongue, skin, etc.
  • The forebrain consists of Cerebrum, thalamus, and hypothalamus.
  • The cerebrum is biggest part of brain and is associate with higher function of thinking and actions.
• Midbrain is a small central part of brainstem, developing from middle of primitive or embryonic brain, hypothalamus is a portion of brain
  • It contains a number of small nuclei with a variety of functions, and  lower part of brainstem, comprises cerebellum, pons, and medulla oblongata. 
  • Midbrain connects forebrain and hindbrain. 
  • It is a bridge to transmit signals.
  • The functions are vision, hearing, temperature regulation, and alertness, etc.
• The hindbrain controls heart rate, breathing, blood pressure, sleep, and walking, etc.
  •  cerebellum is part of hindbrain.
  • It is maintaining equilibrium, body balance, and posture during walking, riding, etc.