The correct answer is option 3, S.P. Gupta v. Union of India.

Locus standi refers to the legal standing or the right of a person to bring a case before a court. It determines whether a person has sufficient interest or stake in a matter to be allowed to initiate legal proceedings.

In the case of S.P. Gupta v. Union of India, the Supreme Court of India expanded the scope of locus standi and liberalized the rules governing who can approach the courts. This case involved a public interest litigation challenging the transfer of a Chief Justice by the President based on the advice of the Prime Minister.

The judgment in this case held that any person, whether directly affected by a matter or not, can approach the court to seek redress for violation of constitutional rights or for the enforcement of legal obligations. This broadened the concept of locus standi and allowed individuals or organizations to bring cases before the court even if they were not directly impacted by the issue at hand.

Overall, the case of S.P. Gupta v. Union of India played a significant role in allowing more individuals and organizations to access the courts and seek justice.