The one variable that stands out as the most significant explanation of large variations in living standards around the world is

a. population.

b. preferences.

c. prices.

d. productivity.

Letter d is correct. Productivity.

Explanation:

Living standards around the world are directly influenced primarily by a country's productivity.

In macroeconomics, an increase in productivity means a strengthening of the exchange rate and an increase in employment.

In microeconomics higher levels of productivity in a country translate into increased consumption and increased purchasing power of the individual.

Therefore one economic theory impacts directly on the other, and as productivity increases in one country, the consequences are improved living standards for the population as a whole.