One technique used by stalin to strengthen his control of the soviet union was a making alliances with the powerful kulaks b methodically purging potential rivals and enemies c placing supporters of trotsky in leadership positions d granting communist party members immunity from arrest

The correct answer is b methodically purging potential rivals

In these purges millions of people died. Many of them would just disappear over night while other would be sent to Siberia into Gulags or to work until they die in mines and similar projects. He would even remove them from history by removing them from pictures and destroying everything they ever had or wrote or built.

The answer is option e.