The correct answer is “demand from developing countries”.
The reason that most likely caused a steady increase in price per barrel of oil between 2001 and 2008 was the demand from developing countries.
In 2001, the normal price of a barrel of oil in the United States was $23.- And with the inflation-adjusted price, $31.87. In 2008, the normal price of a barrel of oil in the United States was $91.- and with inflation-adjusted price, $113.- In is considered that the demand from developing countries could have been a reason for the steady increase in the price of a barrel.
The other options of the question were, a) a global recession, b) scarcity of petroleum, and c) a large population growth.