I think is C because in the text it's the main idea.
An allusion. Religious allusion to be more specific.
D False Study is the correct answer
I personally would say ancient castle in ruins...
My reason for this would be that the office building and the apartment in the city will be modern and won't have a Gothic feel to it...
The mansion in the countryside will be surrounded by green fields and well that scenario doesn't sound very dark and Gothic to me...
And well that castle in ruins will have a spooky side to it as it will be abandoned and especially at night will be a host to a whole range of spooky thoughts and ideas.