The sentence refers to the person "he", emphasizing that he personally said it.
The correct reflexive pronoun "himself" is used to emphasize the subject "he".
The correct sentence is:
"He himself said so."
Thus, the correct answer is Option A ("He himself said so").
Among scientists, the discovery of the double helix structure of DNA and the genetic code it incorporates is widely regarded to be one of the most significant scientific discovery of the twentieth century.