I believe the correct answer is D. "'If Jimmy is alive on earth, he’ll be here by that time.'"
It tells us that Jimmy might be a good man, at least by the impression of his best friend, who hasn't seen him twenty years. Jimmy seems to be an honest man who keeps his word. The C option may look to be the best answer, but in my opinion, it is not: it is far too ambiguous: Jimmy doesn't go away with the intention to protect his friend. All he wants is to send another cop to arrest Bob, because he can't face his old friend and put him under arrest. He even asks Bob how long is he planning to be there, which is also ambiguous: maybe he wants to protect him, and maybe he just intends to keep an eye on him.