Step 1: Understanding the Historical Context
The question asks for the official ground on which the National Emergency was declared in India in 1975 by the government of Prime Minister Indira Gandhi.
Step 2: Constitutional Provisions (Then and Now)
Under Article 352 of the Constitution, a National Emergency can be declared if the security of India is threatened.
At the time of the 1975 emergency, the grounds for declaration were "war, external aggression, or internal disturbance".
The government of Indira Gandhi invoked the "internal disturbance" clause to declare the emergency.
After this controversial use, the 44th Constitutional Amendment Act of 1978 changed the wording. The phrase "internal disturbance" was replaced with "armed rebellion". This was done to make the clause more specific and prevent its misuse.
Step 3: Evaluating the Options
(A) War and (B) External Aggression: These are valid grounds for an emergency, but they were not the reasons cited in 1975. Emergencies on these grounds were declared during the wars with China (1962) and Pakistan (1971).
(C) Armed Rebellion: This is the current ground for an emergency, but it was not the ground in 1975. It replaced "internal disturbance".
(D) Internal disturbance: This was the specific, and now-defunct, constitutional ground used for the declaration of the 1975 National Emergency.
Step 4: Final Answer
The 1975 National Emergency was declared on the ground of "internal disturbance". Therefore, option (D) is the correct answer.