The following sentences test correctness and effectiveness of expression.

The African tsetse fly does not need a brain, everything it has to do in life is programmed into its nervous system.

The correct option is C Brain, for everything
Here the two parts are linked by a comma but there seems to be no real connection between them: they exist independently. They need to connected by 'for' as it indicates purpose which is what the question requires. Only option C rectifies this error without adding any new ones.