The Soviet press and government were not stunned nor disgusted upon the discovery of the Concentration camps. Soviet soldiers were probably so familiar with the horrors of the Nazi system by then that they didn't think them particularly appalling. Equally depressing things had already happened within the Soviet Union in the war on the eastern front.
A Historian would look for a primary source (option d. ) - the definition of a primary source is that it was written at the time of the event, and not later (this is the secondary choice).
