• Japan's population will more than halve, from a peak of 128 million in 2017 to less than 53 million by the end of the century. Japan already has the world's oldest population and the highest rate of people over the age of 100. • The Italian population is also expected to more than halve, from 61 million in 2017 to 28 million by the end of the century. Like Japan, Italy is known for its ageing population. More than 23% of people there were over the age of 65 in 2019, according to World Bank data. • Nigeria and other countries in Africa are bucking the trend of population decline. Nigeria will become the world's second most populous country, with 791 million people. The new study predicts that Nigeria will have one of the largest working-age populations in the world by 2100 and see big increases in GDP. • South Korea's demographic crisis has become the top risk to economic growth and the social welfare system, with the country's population of 51 million on track to halve by the end of this century. South Korea has previously projected its fertility rate is likely to fall further to 0.68 in 2024. Hence option (a) is the correct answer.