On 1 January 2017, who assumed the post of Secretary-General of the United…

Concept

The Secretary-General of the United Nations is the head of the UN Secretariat, appointed by the General Assembly on the Security Council's recommendation for a five-year term. When a term ends on schedule (rather than the SG leaving mid-term through death or resignation), the successor's tenure begins on 1 January of the following year. To identify who took office on a given date, find whose scheduled term commenced exactly on that date.

Application

The ninth Secretary-General assumed office on 1 January 2017 for a five-year term, succeeding the eighth Secretary-General whose tenure ran through 31 December 2016. The official who began this term on 1 January 2017 was Antonio Guterres, the former Prime Minister of Portugal and former UN High Commissioner for Refugees.

Cross-check

This is fixed by the handover date rather than by general prominence:

• Antonio Guterres - took office on 1 January 2017; this matches the date asked.

• Ban Ki-moon - the immediate predecessor, whose second term ended on 31 December 2016, the day before.

• Shashi Tharoor - an Indian diplomat and politician who contested the 2006 race but was never Secretary-General.

• Kristalina Georgieva - a Bulgarian economist associated with the World Bank and later the IMF, not the UN Secretariat.

Only the figure whose term started on 1 January 2017 satisfies the question, so the answer is Antonio Guterres.