The right option is C.
"Don wore braces on both of his legs; still, he was an avid runner".
In this sentence, "still" indicates a sort of surprise. It is placed between a semicolon and a comma because it functions as a transition word.
Transition words show the relationship between sentences and help the text to be cohesive. Some examples of transition words are "nevertheless", "however", and "even though" among others.