If a new State of the Indian Union is to be created, which one of the following Schedules of the Constitution must be amended ?

The First Schedule of the Constitution lists the States and Union Territories of India and thereby defines the territory of each State; Article 1(2) explicitly states that the States and the territories thereof shall be as specified in the First Schedule [1]. Creation or reorganisation of States therefore requires altering that formal list so that the new State (or changed boundaries) appears in the Schedule. Reorganisation statutes historically amend the First Schedule entries to reflect newly formed or altered States, so the First Schedule must be amended when a new State is created [2].

Sources

1. [1] https://www.mea.gov.in/Images/pdf1/Part1.pdf
2. [2] https://www.mea.gov.in/Images/pdf1/S1.pdf