Zack Pled Not Guilty by Reason of Insanity to Killing

Zack pled not guilty by reason of insanity to killing his parents and his brother. He was arrested after their deaths while walking down the street covered in dried blood and not wearing any clothes. He had always been an introvert, and people used to call him names and say that he was crazy. Over couple of years leading up to the murders, Zack had a lot of blackout-type situations where he found himself somewhere unfamiliar doing something strange and didn't remember the days before he got there. Zack then spent the better part of two years in a locked psychiatric facility awaiting trial. He feels ill from the medicine they make him take, he doesn't recognize the man that they say is his attorney, and he cannot remember his full name. Zack fires his attorney, or, at least he fires the man who they say is his attorney. An interpretation of which amendment grants him the right to defend himself pro se ?

A) Fourth Amendment
B) Eighth Amendment
C) Sixth Amendment
D) Fourteenth Amendment