Final answer:
Sergei Kirov was shot by an unknown assailant at the Communist Party headquarters in Leningrad in 1934. His assassination precipitated Stalin's Great Purge and heightened repression in the Soviet Union.
Step-by-step explanation:
The assassination of Sergei Kirov, a prominent Soviet politician, occurred in 1934. Kirov was shot by an unknown assailant at the Communist Party headquarters in Leningrad (now St. Petersburg). The case b) describes the correct circumstances surrounding Kirov's death. This assassination triggered a massive and brutal campaign during the Great Purge, as Stalin used Kirov's murder as a pretext to eliminate real and imagined opposition within the party and society at large. The aftermath strengthened Stalin's grip on power and led to increased repression within the Soviet Union.