It has been said that Thomas Jefferson, as president of the

I am pretty confident it's E, and here's why.
A. get rid of the "as president of the United States" and reread. It has been said that Thomas Jefferson was brilliant at using political power in a way...
... his brilliance should suggest the way way he used the power, whereas in this case, it comes off as he was brilliant at using power in a way, in one way among many ways. it doesn't seem to relate to THE way mentioned in the sentence- using it so discreetly, if u will.
B. clearly wrong because it ends abruptly and meaninglessly.
C. clearly wrong, although clever at its attempt to trap those for whom "president of the United States" stuck out most about the sentence because of the phrase's careful placement within commas, forcing them to pause their reading flow, and instead decide that the topic at hand is presidency and not the way he used power.
D. Again, clearly wrong because people were unaware of what?
E. "in such a way that" is the correct usage. "his using it" is also correct. i think it's called gerund or something. read about it. it gets thrown around pretty frequently.