Answer : 2. "Russia"
Explanation :

The Indian Air Force has successfully operated three of its S-400 air defense missile squadrons along its borders with China and Pakistan. In 2018–19, India agreed with Russia to purchase S-400 missiles worth ₹35,000 crore. Of the five squadrons that were to be delivered, three have been delivered. While the delivery of the remaining two has been disrupted due to the Russia-Ukraine conflict.