The correct answer is - c. The Soviets invaded Hungary, stopped reforms, and later executed Nagy.
The Kaposvar born Hungarian politician Imre Nagy has a special place in the Hungarian history. He was a communist, and was the Prime Minister of Hungary from 1952 to 1955, as well as 1956. Despite being a communist, Nagy was not close to the Soviets and their policies, and he wanted them out of his country. Nagy initiated a revolution against the Soviet based government and he tried to push out the Soviets and their marionettes out of Hungary.
The revolution ended up tragically. The Soviet Union furious about what was happening in Hungary invaded the country. Lot of people were killed, and the revolution was stopped. Nagy was executed two years afterwards.