The correct answer is (c) Six months.
The Constitution of India provides that Parliament must meet at least twice a year. The first session must be held within six months of the previous session, and the second session must be held within twelve months of the previous session.
The President can summon a joint sitting of the two Houses of Parliament if there is a disagreement between them on a bill. The President can also prorogue (end) a session of Parliament.
The maximum possible gap between the two sessions of Parliament is six months. This is because the Constitution requires that Parliament must meet at least twice a year, and the first session must be held within six months of the previous session.