Explanation:

- The executive authority of the Union is defined in Article 53 of the Indian Constitution.

- Prime Minister: Not correct. The Prime Minister is the head of government but not the executive authority holder by the Constitution.

- President: Correct Answer. The President is the constitutional head of the Union executive. All executive actions are taken in the President’s name.

- Cabinet: The Cabinet (Council of Ministers) actually wields power but constitutionally advises the President.

- Union Legislature: The legislature (Parliament) makes laws and does not exercise executive authority.