The correct answer is option 3, U.S.S.R. In the year 1966, the U.S.S.R (Union of Soviet Socialist Republics) offered its good office to resolve the Indo-Pak conflict peacefully. The U.S.S.R played a significant diplomatic role during this period in international affairs. The U.S.S.R expressed its willingness to help mediate and find a peaceful resolution to the conflicts between India and Pakistan. It is important to note that although other countries like the U.S.A, Britain, and Japan have also been involved in diplomatic efforts between the two countries, the U.S.S.R was the specific country that offered its good office in 1966.