Option D (all of the above) is correct. The provided context confirms all three functions: (A) Thinking - The context states 'The fore-brain is the main thinking part of the brain' and 'The brain thus allows us to think and take actions based on that thinking.' (C) Balancing the body - The context states that the cerebellum (part of hind-brain) 'is responsible for precision of voluntary actions and maintaining the posture and balance of the body.' (B) Regulating the heart beat - The context describes the brain as 'the main coordinating centre of the body' that 'receive information from all parts of the body and integrate it.' As the main coordinating center, the brain is responsible for regulating involuntary functions like heart beat. Therefore, all the given options are correct.