Why not det?
"Vad betyder det?" means "what does it mean?", not "what does this mean?"
Is there a difference between betyder and menar?
I think "menar" is someones opinion and "betyder" is the meaning of something. I don't know if this makes sense to other people than me :p
Why is "vad betyder den här" not recognised as correct? Is there a rule for that?
I believe it's that if there is no specific noun being referred to, det is always used, just like in det regnar.
Why is "vad betyder detta?" the correct answer here?
this is det här or detta. There's no real difference in meaning between the two; detta may be slightly more formal.
yes why not menar?