The correct answer is "But."
Explanation:
The word "But" serves as a transition that indicates contrast between two ideas or statements. In this passage, it contrasts Dr. Koch's usual lecturing style (described as thin, reedy, and filled with "ums" and "ers") with the clarity and elegance of his presentation on March 24, 1882. This suggests that, despite his typical shortcomings as a lecturer, he was able to convey his findings effectively on that particular day. Thus, "But" signals the contrast between his usual speaking manner and the effectiveness of his presentation in this instance.