I think the first "that" refers to Byron possessing powers of observation second "that" points to Byron becoming a great anthropologist because of his powers of observation.
If this is right, then the answer should be D.
I am planning to take the test by end of march. Is it possible to get the 1000 SC, CR questions from somewhere. I have started preparing this week ... What is the best strategy I can adopt. I am weak in the verbal section.