A national border would most likely have the most protection because nation's seek to prevent the import of illegal goods, movement of individuals into the country, and also to protect their borders against potential foreign invasion. Traditionally national borders would have more defenses, army, security forces, and police to protect the border than any of the rest of these borders.
A border is a limit that divides two territories of different nature, the borders divide different territorial entities such as continents, countries, cities, states, departments, neighborhoods, private properties, among others. However, the borders that have greater protection and security presence are the borders of the countries, because governments want to maintain their state sovereignty throughout the national territory and this is one of the most common actions worldwide. So, the correct answer is A. State
