The correct option is B War, external aggression or armed rebellion.
An emergency under Article 352 of the Constitution of India can be declared by the President only during war, external aggression or armed rebellion. Such a proclamation of emergency has to be approved by both the Houses of Parliament by absolute majority of the total membership of the Houses as well as 2/3 majority of members present and voting. Once proclamation of emergency approved by the Parliament, the emergency remains in force for a period of six months from the date of proclamation.