What term was given to the lessening of tensions between the united states and the soviet union during the 1960's and 1970's

The term used to refer to the lessening of tensions between the U.S and USSR during the 1960's and 1970's is called détente.

The best and most correct answer among the choices provided by your question is the third choice or letter C. 

After the Congress of Berlin, Austria annexed Bosnia and Herzegovinia.
Congress of Berlin, (June 13–July 13, 1878), diplomatic meeting of the major European powers at which the Treaty of Berlin replaced the Treaty of San Stefano, which had been signed by Russia and Turkey (March 3, 1878) at the conclusion of the Russo-Turkish War of 1877–78.

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