The human brain is amazing big intricate and delicate. The brain is a very complex organ
Hmm, I would probably go with B. While both I think are correct, B makes a little more sense.
The only sentence here with an infinitive phrase is the first one, and the infinitive phrase there is 'to write'. You ask the question - What did Christa choose? in order to find the object of that sentence, and the answer is 'to write'.