The correct answer is - internal and external pressure.
Boris Yeltsin started off as a very popular figure in Russia, but that quickly changed. He made a lot of reforms, lot of which with good intention, but heir implementation was terrible. That mostly affected the economy of Russia. The lived in poverty, and the country was brought to an economic collapse. Also, the corruption was very wide spread, and the mob was running most of the things in the country as it had its hands untied.
After a lot of pressure from the Russian people, as well as the international community, Boris Yeltsin resigned. He left the country in the hands of Vladimir Putin who was prime minister at that time.