Answer:
D) Yes, Robert has assumed only Jack's title, which is no title at all
Explanation:
Jack never owned the music box, he just took it to the repair shop. No title was ever passed form Harry to Jack and Jack didn't even have the intention to take the music box for himself.
In this case, Jack messed up, and he cannot get the music box back, but once he tells Harry, then he can claim it back. Even if Robert was a buyer in good faith, it doesn't matter because he cannot purchase something from someone that doesn't own it.