Toulouse is known as the pink city because A. many of the buildings are of a kind of a pink color.
Many buildings in Toulouse are pinkish, terracotta colored. They don't have to be painted pink there, it is just that historically, people thought that color was the most pleasing, and thus they continued painting the buildings pink, which is why that is the predominant color there at the time being. C and D are also incorrect - French people aren't generally red-headed, and they prefer wearing dark colors.