The correct answer is: B. once a year.
The General Assembly is the main deliberative organ of the United Nations. It is composed of all 193 Member States of the United Nations. The General Assembly meets in regular sessions every year from September to December. It may also hold special sessions at the request of a majority of its members or of the Security Council.
Option A is incorrect because the General Assembly does not meet once a month. It meets once a year in regular sessions and may also hold special sessions at the request of a majority of its members or of the Security Council.
Option C is incorrect because the General Assembly does not meet twice a year. It meets once a year in regular sessions and may also hold special sessions at the request of a majority of its members or of the Security Council.
Option D is incorrect because the General Assembly does not meet after every three months. It meets once a year in regular sessions and may also hold special sessions at the request of a majority of its members or of the Security Council.