What is characterized by self-sustaining growth and its spread across an entire country?

The phase characterized by self-sustaining growth and its expansion across an entire country is best understood through the concept of "Drive to maturity." During this stage of economic development, a nation experiences an increase in productivity due to more diversified industry and improved technology. This means that the economy can grow independently of foreign influence, with local industries becoming robust and sustainable over time.

In this phase, the economy shifts from preliminary growth to a more advanced stage where various sectors contribute significantly to overall production. This leads to improved living standards and higher levels of income for citizens, facilitating widespread access to goods and services.

The other options represent different stages or types of economic structures. A traditional economy relies heavily on agriculture and is often limited in growth potential. The age of mass consumption follows the drive to maturity and is characterized by a higher demand for goods and services but does not focus primarily on growth across the entire country. A post-industrial society typically denotes a shift toward service-oriented industries, rather than the self-sustaining growth that the drive to maturity encompasses. This makes "Drive to maturity" the most appropriate answer in this context, reflecting the robust, widespread economic activity expected during this phase.