The correct answer is: A. International Volunteers Day
International Volunteers Day is a United Nations observance held annually on 5 December. It was established in 1985 by the United Nations General Assembly to recognize the efforts of volunteers in promoting peace and development.
World Soil Day is an international observance held annually on 5 December to raise awareness of the importance of soil and to promote sustainable soil management. It was established by the Food and Agriculture Organization of the United Nations in 2002.
Therefore, 5 December is celebrated as International Volunteers Day, not World Soil Day.