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.
Explanation: Gothic literature is a style of writing that has elements of horror, death and fear, it can also include romance. Usually the reader of this genre experiments strong emotions of fear and suspense. The settings of a gothic work are usually desolate landscapes, castles with dungeons, ancient buildings in ruins, secret doors, etc. So, from the given options, the one that best represents the setting of a gothic work is an ancient castle in ruins.
i'd say it's okay to tell ghost stories to children as long as they aren't too graphic. Ghost stories can be fun to tell around the campfire and stuff.
