When Germany began World War II in 1939, a secret agreement with U.S.S.R allowed Soviet troops to occupy eastern Poland. The correct option among all the options given in the question is option "a". The Treaty of Non-Aggression between germany and Soviet union gave this advantage to the Soviets. A part of this treaty had a secret protocol included in it that divided the territories of Poland, Romania, Finland, Latvia, Estonia and Lithuania into German and Soviet spheres of influence. The Soviets were sure that the Germans would not attack them for sure and so they were comfortable with this treaty.