The correct answer is A) they allowed foreign armies access to Northern India.
The mountain passes influenced the rise and decline of ancient Indian Empires in that they allowed foreign armies access to Northern India.
Mountains served as a natural barrier to protect cities in ancient times. The problem with these natural barriers such as mountains is that many times they have passes that were used by the army of the enemies to arrive and invade cities. That was the case of India. But Indian mountain passes such as Khyber pass or Nathu La served as an exit for Indian Empires to leave and explore or trade with other territories.