The correct answer is A) the movement of the Soviet Union troops into Afghanistan.
What caused the Cold War to re-escalate following the SALT 1 treaty was the movement of the Soviet Union troops into Afghanistan.
The Russian Army invaded Afghanistan in December 1979. The motive, ti impulse the Communist government of the Peoplés Democratic Party in Afghanistan. The USSR deployed 100, 000 soldiers in the following months but were encountered with heavy resistance by the Muhajideen, that was supported by the United States. This incident elevated the tensions between the Soviet Union and the United States during the Cold War.