The correct answer is B : to ensure the Soviet military would not interfere with the German invasion of Poland.
The Molotov-Ribbentrop Pact, also known as the Treaty of Non-aggression between Between Germany and the Soviet Union, was a neutrality pact signed on August 23, 1939 in Moscow.
The Pact provided a written guarantee of non-belligerence (interfering) by each party towards the other, meaning that neither government would aid an enemy of the other party.
Germany, not fearing opposition from the Soviets, invaded Poland on September 1, 1939.
The Pact was terminated on June 22, 1941, when the Germans invaded the Soviet Union as part of the Operation Barbarossa.