Certain landmark cases of the late 1970s and 1980s show exactly what the Supreme Court was doing when it loosened the rule of locus standi, and lining up each option against those cases helps identify the right answer.
The cases that actually relaxed locus standi are the founding cases of Public Interest Litigation, so that is the development being described.
So, the correct answer is the evolution of Public Interest Litigation.
Constitutional law questions like this one can often be resolved by first asking which specific legal dimension each option belongs to, standing, meaning who can sue, jurisdiction, meaning where a court can act, grounds of review, meaning why a decision can be struck down, or remedy, meaning what a court can order. Sorting the options along this dimension makes the answer clear.
Once the options are sorted by which legal question they actually answer, only Public Interest Litigation directly answers the who may approach the court question that the relaxation of locus standi is about.
Hence, the correct answer is the evolution of Public Interest Litigation.