The main issue with this argument is that there is no reason for the passage to be describing the portion of the brain that is known to be useless as a "tremendous source of creativity and innovation". The author makes no effort to justify that this unused/useless 90 percent will translate to being a source of creativity or innovation.
C is wrong because the argument did not state that 90 percent of the brain serves no function because "we do not know what this 90% does". Rather it explicitly states that we know it is useless based on the conditions of many people with brain damage.
I hope this helps. Please let me know if you'd like me to elaborate further!