Which sentence correctly uses the word agile?

The answer is letter B. 
The best form of punctuation to grab the reader’s attention and to maintain a correct grammar is letter B: Extreme runners must be agile, strong, and skilled (not unlike other athletes), but they must also embody courage. 

This sentence construction is more correct because it directly describes the subject in the sentence (e.g. extreme runners) and the character that they must possess in able to be a good athlete. The other part of the sentence was in  parentheses because it shows an indirect comparison to the subject of the sentence.

Look at the passages where the author describes the isolation and insanity that dr frankenstein has