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[Star Wars] When Yoda asks Obi-Wan if Luke will finish what he begins, why doesn't Obi-Wan answer?

Does Obi-Wan have a suspicion that Luke will do the Bespin thing and not want to either lie to nor tip off Yoda?
gpt
Don't forget that Obi Wan is dead.

His spirit is still there in order to advice both Yoda and Luke. He is not here to push Yoda into training Luke.

You should see it like he represents the pros and yoda the cons.
Re-watch the scene with that idea in mind, it gets clearer. 

In order to summarize, he doesn't answer because Yoda doesn't really need him to. It's more like a rhetorical question to himself.