Stalin signed a nonaggression pact with Germany because he and Hitler wanted to divide up eastern Europe. The correct option in respect to all the options given in the question is option "b". Actually Stalin wanted time to rebuild his army and Hitler did not want a two front war knocking in his door. This was the main reason behind the pact and secretly they made a pact that they would divide up Eastern Europe later on. It is true that both of the leaders knew that this pact would not stand for long, but both of them had their own reasons for signing such a pact. Hitler could then attack Poland without the fear of Russia interfering.
