Which Secretary General of the United Nations was the only one to die while still in office?

Introduction / Context:
This general knowledge question tests awareness about the history of the United Nations and specifically about its leadership. Knowing which Secretary General died while still serving in office highlights both an important biographical fact and a key historical incident in UN history, involving a plane crash during a peace mission in Africa.

Given Data / Assumptions:

• The post mentioned is that of Secretary General of the United Nations.
• The question asks for the only Secretary General who died during his tenure and not after retirement.
• The options list several well known UN Secretaries General from different periods.

Concept / Approach:
To answer correctly, we must recall the chronological list of UN Secretaries General and identify who died in office. Dag Hammarskjold of Sweden, the second Secretary General, died in a plane crash in 1961 while on a peace mission in the Congo. Other Secretaries General such as Trygve Lie, U Thant, Kurt Waldheim and later office holders completed their terms and died after leaving office. Therefore, the correct choice is the name that matches this historical fact.

Step-by-Step Solution:
Step 1: Recall that Dag Hammarskjold was the second UN Secretary General, serving from 1953 to 1961. Step 2: Remember the historical incident in which he died in a plane crash near Ndola in Africa in 1961 while travelling on a peace mission related to the Congo crisis. Step 3: Note that other Secretaries General in the options, such as Trygve Lie, U Thant and Kurt Waldheim, all completed their terms and later died as private citizens. Step 4: Conclude that Dag Hammarskjold is the only Secretary General who actually died while still holding the office.

Verification / Alternative check:
A quick cross check with standard UN history references or reputable encyclopedias confirms that Dag Hammarskjold died in 1961 in a plane crash and that no other Secretary General passed away in office. All other names listed in the options resigned, retired or completed their terms before their deaths, which occurred later. This independent verification supports the conclusion that Dag Hammarskjold is uniquely correct.

Why Other Options Are Wrong:
Trygve Lie: He was the first UN Secretary General and resigned in 1952; he did not die in office. Kurt Waldheim: He served from 1972 to 1981 and later became president of Austria; he died long after his UN term ended. U Thant: This Burmese diplomat completed his tenure and died several years after leaving office. Ban Ki moon: He completed his terms as Secretary General and is very clearly not someone who died in office.

Common Pitfalls:
Many learners confuse different Secretaries General or assume that an older sounding name implies death in office. Another mistake is mixing up U Thant and Dag Hammarskjold because both served in the early decades of the UN. Some candidates may also answer based on guesswork rather than specific historical knowledge. It is important to focus on the unique event of a fatal plane crash during a UN mission when remembering this fact.

Final Answer:
The only Secretary General of the United Nations to die while still in office was Dag Hammarskjold.