A. 1950s is not correct.

During the 1950s, the Security Council approved only two peacekeeping missions. The United Nations Emergency Force was authorized in November 1956 to oversee the truce between Egypt and Israel following the Suez Canal crisis. In 1958, with Lebanon about to erupt into civil war between Christian and Muslim factions, a UN Observation Group was set up to make sure no foreign fighters or arms shipments were crossing the border into the country.

The first ever peacekeeping mission was authorized by the Security Council in 1948 to supervise a cessation of hostilities between Israel and Arab nations in Palestine. May 29, the anniversary of the authorization of that mission, is now observed as the International Day of United Nations Peacekeepers, which honors those who have served in, and lost their lives in, peacekeeping operations.

B. 1980s is not correct.

During the 1980s, the UN authorized six new peacekeeping missions, all of them in the last few years of the decade when Cold War relations between the United States and the Soviet Union were beginning to thaw.

The first new mission of the decade was set up in 1988 and authorized UN peacekeepers enforce international agreements between Afghanistan, Pakistan, the United States and the Soviet Union following the withdrawal of Soviet forces after its nine-year war in Afghanistan. Later that year, the UN Peacekeeping Forces were awarded the Nobel Peace Prize.

C. 1990s is correct.

With the collapse of the Soviet Union in 1991, the Cold War came to an end. In a new spirit of cooperation, the United Nations Security Council was able to authorize more and larger deployments of peacekeeping troops.

Thirty-five new peacekeeping missions began in the decade — more than three times the number in any other decade — and more than half the total number of missions (67) in the history of UN peacekeeping operations. UN peacekeepers were involved in the aftermath of the U.S. liberation of Kuwait in 1991, the implementation of peace accords ending the Rwandan civil war, and several missions in the Balkans.

D. 2000s is not correct.

The 2000s were the second busiest decade for UN peacekeeping missions, with ten new missions authorized. Seven of the ten sent peacekeepers to mitigate armed conflicts or to implement peace accords in Africa, including the Eritrean-Ethiopian war in 2000, the civil war in Burundi in 2004 and the Darfur conflict in 2007.

So far in the 2010s, the United Nations has authorized four new missions — three in Africa and one in the Middle East. The newest is the UN Supervision Mission to Syria, established in April 2012, to monitor a cessation of armed violence relating to the country's "Arab Spring" uprising.