Who was the second Secretary General of UN?

The correct option is Dag Hammarskjold.

To understand why this is correct, we can look at the chronological order of the Secretaries-General of the United Nations (UN):

1. Trygve Lie (Norway): Served as the first Secretary-General from 1946 to 1952.
2. Dag Hammarskjöld (Sweden): Served as the second Secretary-General from 1953 until his tragic death in a plane crash in 1961.
3. U Thant (Myanmar/Burma): Served as the third Secretary-General from 1961 to 1971.
4. Kurt Waldheim (Austria): Served as the fourth Secretary-General from 1972 to 1981.

Therefore, Dag Hammarskjöld was indeed the second Secretary-General of the United Nations.