No Change Expected
The correct answer is C: there will be 2 billion children in 2100. The number of children in the world will not increase. It will remain stable. The UN experts publish new official population forecasts every second year in their publication World Population Prospect. They work with multiple alternative scenarios. The one they think is most probable is called the ‘medium fertility variant’, which falls between the highest and lowest predictions of fertility and mortality decline worldwide. For the past ten years, the UN Population Division has published forecasts of this scenario predicting that the number of children in the year 2100 will not be higher than it is today.
In their latest revision published in 2017, the UN Population Division estimates that there are 1.975 billion children (aged 0 to 14) in 2017 and forecast that the number will be 1.957 billion in 2100. The number of children worldwide is expected to peak at 2.094 billion in the year 2057).
Check out United Nations Population for more details.