The correct answer is: C. five years.
The UN Secretary-General is appointed by the General Assembly upon the recommendation of the Security Council. The term of office is five years, and the Secretary-General may be reappointed for a second term.
The Secretary-General is the chief administrative officer of the United Nations. He or she is responsible for the day-to-day running of the organization, and for representing the UN to the outside world. The Secretary-General also plays a key role in promoting peace and security, and in developing and implementing the UN’s work in other areas, such as economic development, social progress, and human rights.
The current Secretary-General is António Guterres, who was appointed in 2017. He is the ninth person to hold the position.