Final answer:
The United States is the country that will not experience a significant decline in population by 2050, with an expected growth of about 26 percent.
Step-by-step explanation:
According to the information provided, the country that will not experience a significant decline in its population by 2050 is the United States. While low-fertility countries like China are expected to see population declines of approximately 20 percent, the United States, which falls into the intermediate fertility category along with India, is actually expected to experience a growth of about 26 percent. Moreover, the U.S. will remain the third most populous nation with its population increasing from 308 million in 2010 to 423 million by 2050. In comparison, countries such as Japan will experience a significant decline in population, making the United States the correct answer to the question.