The correct answer is (c) six months.
Article 249 of the Constitution of India deals with the power of Parliament to make laws with respect to any matter in the State List in the national interest if a resolution to that effect is passed by both Houses of Parliament. The resolution passed under clause (1) of Article 249 shall remain in force for a period not exceeding six months, unless before the expiry of that period, it is approved by resolutions passed by the Houses of the Legislature of the State concerned.
The other options are incorrect because they are not mentioned in Article 249.