Step 1: Purpose of the Ninth Schedule.
The Ninth Schedule was added by the First Constitutional Amendment (1951) to protect land reform laws from being struck down on the grounds of violation of fundamental rights.
Step 2: Article 31B and its effect.
Article 31B provides that any law placed in the Ninth Schedule cannot be void on the ground of contravention of Part III. Thus it “validates” laws even if they were previously unconstitutional.
Step 3: Judicial Position (I.R. Coelho Case, 2007).
The Supreme Court later held that the Ninth Schedule is subject to basic structure review, but the function of the Schedule—to validate laws—remains intact.
Conclusion:
Since the question asks which schedule deals with validation of acts, the correct answer is Schedule IX.