The question seems to be based on a misunderstanding. Adolf Hitler did not join the Soviet Union; rather, he signed the Molotov-Ribbentrop Pact (a non-aggression treaty) with the Soviet Union in 1939. The goal of this pact was to avoid a two-front war for Germany and to allow for the partition of Eastern Europe between Germany and the Soviet Union.
In terms of the options provided:
A. To spread Pro-Soviet ideologies throughout Germany - This is incorrect, as Hitler's ideology was fundamentally opposed to communism.
B. To gain political power - This is somewhat misleading in the context of the treaty; it was more about strategic military and territorial goals.
C. To strengthen Germany's border defense at Rhineland - This option is not directly related to the pact, as the Rhineland had already been remilitarized by Germany in 1936.
D. All of the above - Given the inaccuracies in the other statements, this option is also incorrect.
In summary, the premise of the question is flawed, as Hitler did not join the Soviet Union. He engaged in a strategic treaty with it for military and territorial purposes.