Explanation:

- The 70th Report of the Law Commission dealt with the reform of grounds for dissolution of marriage and judicial separation under the Hindu Marriage Act, but it did not recommend the “Breakdown Principle” directly.

- The 71st Report specifically addressed the introduction of the “Breakdown Principle” as an additional ground for divorce. This principle acknowledges that a marriage which has irretrievably broken down should be dissolved.

- Correct Answer: Option 2 – 71st Report

- The 72nd Report dealt with other aspects of matrimonial laws but did not focus on introducing the Breakdown Principle.

- "None of the above" is incorrect as the Breakdown Principle is indeed covered in one of the options provided.