The correct answer is (a).
Article 355 of the Indian Constitution states that it shall be the duty of the Union to protect every State against external aggression and internal disturbance and to ensure that the government of every State is carried on in accordance with the provisions of this Constitution.
The Supreme Court held that the Illegal Migrants Determination Act, 1993 was unconstitutional on the ground that it violated the moral obligation of the Centre under Article 355 to protect the State of Assam from illegal immigration. The Court held that the Act was discriminatory and violated the fundamental rights of the people of Assam.
The other options are incorrect. Article 356 of the Indian Constitution deals with the imposition of President’s rule in a State. Article 256 deals with the executive power of the Union. Article 257 deals with the relations between the Union and the States.