Hello HIrashin,
All of those are passable - I mean none of them break any rules of grammar or logic or anything. But some of them sound better than others, so it starts to get a bit subjective. If you are making this into a test question it might be good to take a different approach since any of the answers are acceptable.
There is nothing grammatically wrong with sightseeing sights, or sightseeing places, or sightseeing areas. There is a redundancy in "sightseeing sights", but that doesn't make it incorrect, it just makes it a bit clunky.
If you gave that question to 10 different English speakers, and told them to arrange the options in order from best to worst, you would get 10 different answers. Since brevity is usually considered the core of good writing, your best bet would probably be "a", but calling any of the other options incorrect would be overstating the case, I think.