D. Religion spreading through the Silk Road.
A and B are similar but, I would say the best answer is A.
The Collapse of the Soviet Union was the collapse of Berlin Wall in November 1989 and the dethrone of collectivist rule all over Eastern Europe. President Bush chose to let events unroll in connection with the living matter, careful not to do anything intensify and augment Gorbachev’s position.
Im not exactly sure what the real name is for that but an example would be the queen of england, she has the title and all, but there are a group of people who make the laws. I am pretty sure this would be constitutional monarchy.