Roughly speaking each of the options translates into
A. Stalin Leader and Better defensive Performance
It was Stalin's paranoia that led him to purge the Red Army. Once the threat was identified the Officer Corps rallied around their country's survival and delivered a superior performance that the 'Yes' men they were replaced with OTL lacked the mental flexibility to provide.
B. Stalin Leader and Worse Defensive Performance
The intelligentsia of educated military personnel kept their powder dry to see if an opportunity would arise to dispose of this despot and still retain control of their country. Stalin, wise to this, began his purge while the invasion took place.
C. Stalin Deposed and Better Defensive Performance
Following Stalin's failure to acknowledge the invasion of the Soviet Union, the officer corps took swift action to replace him with a figure that was capable of repelling the German threat. The early encirclements were greatly reduced and a better led military meant that the Wermacht were more easily repulsed than OTL.
D. Stalin Deposed and Worse Defensive Performance
Stalin executed in a coup, opposing factions fought in a civil war that meant the Germans were able to quickly move to their objectives, possibly favouring one faction above the other as a temporary means to victory.