Step 1: Understanding Article 352.
Article 352 allows the President to declare a National Emergency when the security of India or any part of it is under threat due to war, external aggression, or armed rebellion. These are the only constitutionally valid grounds.
Step 2: Checking Each Option.
(A) Matches the constitutional grounds exactly: war, external aggression, and armed rebellion.
(B) Economic instability is not a ground for National Emergency.
(C) "Internal disturbance" was a ground before the 44th Amendment, but now only "armed rebellion" is allowed.
(D) State disagreement has no relevance to Article 352.
Step 3: Conclusion.
Only option (A) correctly states the valid grounds under Article 352.