The correct option is C of one year at a time
Under Article 352, the President can issue a proclamation of National emergency on the ground of war, external aggression or armed rebellion. The effect of this emergency is that while it is in operation, the life of Lok Sabha of Lok Sabha may b extended beyond its normal term i.e., beyond five years by a law of Parliament. It can be extended for one year at a time and cannot be continued beyond six months if the emergency has ceased to operate or revoked.