Economic growth is

growth in a country's population

growth in a country's gdp

growth in a country's savings

all of the above

I would say all of the above because the scientific definition for economic growth is "an increase in the amount of goods and services produced per head of the population over a period of time".
If that isn't right then it is probably B; growth in a country's GDP because GDP's scientific definition is "Gross domestic product (GDP) is a monetary measure of the value of all final goods and services produced in a period (quarterly or yearly). Nominal GDP estimates are commonly used to determine the economic performance of a whole country or region, and to make international comparisons." which is also a clue for the definition.

Glad I could help! =)