Final answer:
Nigeria is likely to double its population in the shortest amount of time due to its high fertility rate.
Step-by-step explanation:
The country that will double in the shortest amount of time is Nigeria. This can be inferred from the information provided by the United Nations Population Fund, which categorizes nations based on their fertility rates. Nigeria falls into the category of high fertility countries, which are expected to triple their population by 2100. In contrast, countries like China, the United States, and India, categorized as low fertility or intermediate fertility countries, are projected to experience much slower population growth or even declines.