The correct answer is - A. It's population is large.
A nation can have a very large GDP, and it can also have a constantly growing GDP, but that doesn't have to mean that the standard of living is particularly well for most of the population. This especially comes to be the case in the countries that have very large populations, like China and India. China has the second highest GDP in the world, thus one would assume that the people are relatively well situated and have high incomes, but the GDP per capita is far from big, it has been growing constantly, but it is still on a medium level. China has around 1.3 billion people, so when the GDP is divided on the amount of people, it doesn't really come as big. India is another case. It has one of the largest GDP's in the world, and its economy is growing quickly, but the majority of the population has low and very low income, thus large portion of the country is poor. In order for India to be able to come even to some medium standard of living, according to its population, it will need to at least double its current GDP.