The question asks about the first Indian who served as a Judge at the International Court of Justice (ICJ).

1. Understand the context of the ICJ:

   The International Court of Justice is the principal judicial organ of the United Nations, established in June 1945 by the Charter of the United Nations. It settles legal disputes between states and gives advisory opinions on international legal issues.

2. Examine the options given:
   • Mohd. Hidayatullah
   • M.C. Setalvad
   • B.N. Rau
   • R.S. Pathak
3. Identify the correct answer:

   B.N. Rau is the correct answer. He was a prominent Indian lawyer and diplomat who significantly contributed to the framing of the Indian Constitution and served as an advisor to the drafting committee. He was appointed as a judge at the International Court of Justice in 1952 and served until 1953.

4. Rule out incorrect options:
   • Mohd. Hidayatullah: While Hidayatullah was a notable legal luminary in India and served as the Chief Justice of India, he was not associated with the ICJ as a judge.
   • M.C. Setalvad: He was the first Attorney General of India and made significant contributions to legal practice in the country, but he did not serve as a judge at the ICJ.
   • R.S. Pathak: While he did serve as a judge at the ICJ, his appointment came many years after B.N. Rau. He served from 1989 to 1991.
5. Conclusion:

   Given the contributions and timeline of service, B.N. Rau was indeed the first Indian to serve as a Judge in the International Court of Justice, making option <p>B.N. Rau</p> the correct choice.