Under Article 355 of the Constitution of India, it is the duty of the Union to protect States against external aggression and internal disturbance. This means that the Union (central government) has the responsibility to ensure the safety and security of the States in case of any threats or attacks from outside the country or within the country.

Option 1 correctly identifies this duty of the Union. It highlights the importance of the Union in maintaining peace, order, and stability in the country by safeguarding the States from both external aggression (such as war or invasion) and internal disturbance (such as riots or rebellion).

Option 2, allowing the extension of executive powers of the Union to any State, does not accurately describe the duty under Article 355. While the Union does have certain executive powers that it can extend to the States in specific situations, it is not the main duty mentioned in this article.

Option 3, declaring the powers of the legislature of a State to be exercisable by or under the authority of the Parliament, is not mentioned in Article 355. The powers of the legislature of each State are determined by the State`s own constitution and are separate from the authority of the Parliament.

Option 4, allowing the Parliament to include or exclude any caste, race, or tribe into