Instead of a troika coming to power after lenin’s death in 1924, what happened?

Instead of the troika coming, there was a power struggle and Stalin took control of the country. He managed to remove all his political opponents and collectivize the properties which led to a rapid industrialization. There was no troika because Stalin ruled himself with an iron fist. He stayed in that position until he died in 1953. 
D. There was a power struggle between Stalin and Trotsky, and Stalin won.

Ithink is d. california but not sure