Hi there!
This is a question from chapter 14 in Classzone.com
The empires discussed in the chapter are the Chinese, Russian and British empires.
They were unable to remain in control of all their lands because there was an unjust and oppressive rule over their inhabitants, that failed to solve economic, political and social problems, and led to both violent and non-violent actions against it.
In China, Mao Zedong opposed the empire by the arms. In Rusia, Stalin led the revolution against the tsar, and in the British Empire, Gandhi was the leader of a civil disobedience movement to free India.