Right answer:
b. metro
This is the correct answer because it's the more logical one. This is because metro subterráneo translates into underground metro system. In fact, there is a metro system in the city of Madrid, the capital of Spain. So this system is the seventh longest metro in the world, with a total length of 293 km. On the other hand, there is a rapid transit system in the City of New York. Finally:
Hay un metro subterráneo en Madrid y en Nueva York.