The correct answer is: A. Caused by war or external aggression.
Article 359 of the Indian Constitution states that the President can proclaim a state of emergency if he is satisfied that a grave emergency exists whereby the security of India or any part of the territory thereof is threatened, whether by war or external aggression or armed rebellion. During a state of emergency, the President can suspend the operation of any provision of the Constitution, except those relating to fundamental rights. This means that the government can enact laws or issue executive orders that are contrary to Article 19, which guarantees certain fundamental rights such as freedom of speech, freedom of assembly, and freedom of religion.
The other options are incorrect because they do not meet the requirements for a state of emergency under Article 359. Option B, caused by internal armed rebellion, does not meet the requirement of being caused by war or external aggression. Option C, caused by constitutional breakdown, does not meet the requirement of being caused by a threat to the security of India. Option D, caused by financial crisis, does not meet the requirement of being caused by a threat to the security of India.