Step 1: Understanding the Concept:
This question pertains to a fundamental principle of administrative law, often considered the third pillar of natural justice: the duty to give a 'reasoned decision' (\textit{audi alteram partem} - hear the other side; \textit{nemo judex in causa sua} - no one should be a judge in their own cause; and the duty to give reasons). This duty ensures fairness, transparency, and accountability, and allows for effective judicial review.
Step 2: Detailed Explanation:
While the principle was discussed in earlier cases, the Supreme Court comprehensively laid down the law on this subject in S.N. Mukherjee v. Union of India, AIR 1990 SC 1984.
In this landmark judgment, the Court held that:
1. The requirement to record reasons is a principle of natural justice.
2. It is a necessary safeguard that checks the arbitrary exercise of power by administrative authorities.
3. It introduces clarity and satisfies the party against whom the order is made.
4. The duty to give reasons is a rule, and the absence of this duty is an exception. An authority must give reasons for its decision unless a statute expressly or by necessary implication excludes this requirement.
- \textit{State of Orissa v. Dr. Binapani Dei} is a foundational case for \textit{audi alteram partem}.
- \textit{Motilal Padampat Sugar Mills} is a key case on promissory estoppel.
- While \textit{State of Maharashtra v. Jalgaon Municipal Council} also deals with administrative law, \textit{S.N. Mukherjee} is the leading authority specifically on the mandatory nature of giving reasons.
Step 3: Final Answer:
The case that made the requirement to give reasons in administrative decisions mandatory is S.N. Mukherjee v/s Union of India.