How can I tell who is a truth teller and who is a liar?
Here are some hints to get you started. Admittedly, this question isn't that easy.

Part (a): Suppose that there are exactly $n/2$ truth-tellers and $n/2$ deceivers. Show that the deceivers have a strategy in which what a goblin A says about a goblin B depends only on whether they both have the same nature. Draw the conclusions.

Part (b): Pair the goblins up. Show that for some pairwise responses, it is safe to get rid of both goblins. Explain why it is useful to have a majority of truth-tellers.

Part (c): This part should be clear enough.