Step 1: Understanding the Question.
The question seeks the landmark case where the Supreme Court of India legalized living wills and passive euthanasia, subject to specific conditions, marking a significant development in the right to die with dignity.
Step 2: Conclusion.
Thus, (b) Common Cause Vs Union of India, (2018) 5 SCC 1 is applicable as it is the case where the Supreme Court laid down guidelines for passive euthanasia and living wills.