Final answer:
The establishment of the European Union did not happen during 1991; it took place with the signing of the Maastricht Treaty in 1992 and formally came into effect in 1993, thus it is the correct answer for the event that did not occur in 1991.
Step-by-step explanation:
The year 1991 was indeed a period of significant changes in the former Soviet Union; however, the event that did not occur during that year is D) The establishment of the European Union.
The dissolution of the Soviet Union (A) was a definitive event of 1991, bringing an end to decades of the Cold War, marked by the signing of the Belavezha Accords (B). These accords were an agreement that declared the Soviet Union effectively dissolved and established the Commonwealth of Independent States. The election of Boris Yeltsin as president of Russia (C) also occurred in 1991, after he played a crucial role in countering the August coup attempt that year.
It's important to note that the European Union (EU) was formally established later, with the Treaty of Maastricht, signed on 7th February 1992 and came into effect on 1st November 1993. The EU represented a different kind of integration amongst European states, focused on economic and political cooperation.