The correct option is E Fixed
I passage 2, the author states that the new findings Nelson and Bloom refer to suggests that the "brain retains its ability to reorganize itself in response to experience or injury throughout life." Nelson and Bloom further conclude that though much of the brain development occurs in the first few years of life, the brain has not finished adjusting and developing [implied by "far from fixed on its trajectory] all that it could even at the completion of adolescence. In this context, "set in its trajectory" [said with reference to the brain] means fixed on a certain course. Thus 'set' means fixed in the given passage.