Final answer:
Japan had the lowest mortality rate for children under age 5 according to the UNICEF report, attributed to its advanced healthcare and public health measures.
Step-by-step explanation:
According to the UNICEF report titled The State of the World's Children, Japan was among the two countries that had the lowest mortality rate for children under age 5. The lowest infant and child mortality rates are typically found in high-income countries, due to factors such as better healthcare systems, higher nutritional standards, and improved access to clean drinking water. In contrast, countries with the highest mortality rates tend to have less developed health infrastructure and face challenges like poverty, malnutrition, and inadequate sanitation. Japan, being a high-income country with advanced healthcare services and highly effective public health measures, has attained a significantly low mortality rate for children under the age of five. The country's emphasis on maternal and child health, as well as the widespread access to healthcare, contribute to this achievement. Thus, among the options provided, Japan is the country with the lowest mortality rate for children under age 5.