D) Eradicating polio
The United Nations efforts to eradicated polio have been extremely effective, and very little occurrence of polio still exists across the globe. According to information provided by the UN on World Polio Day in October, 2017, only 3 countries in the world (Afghanistan, Nigeria, Pakistan) still had any occurrence of the disease. At that point in 2017, only 12 cases of polio had been reported worldwide in that year, and over 400 million children around the world had received polio vaccinations in the past year.