"Is this place free?"
Translation:Är den här platsen ledig?
In fact, I think "ledig" might also be used in the context of being available too.
den + noun without an adjective means that.
(den + adjective + noun = the)
this can be den här or denna.
Well then, given the lack of context, shouldn't "gratis" have been an acceptable answer?
Why was "Är denna ställe ledig?" marked wrong? What is the difference between plats and ställe?
ställe is an ett-word, so you need detta.
plats and ställe are pretty much synonymous.