Which of the following is NOT a supply-side factor?

The correct choice identifies income levels as not being a supply-side factor because it pertains to the demand side of the economy. Supply-side factors are elements that affect the ability and willingness of producers to supply goods and services. These include aspects related to production efficiency, such as the cost of production, which involves the expenses incurred by firms to create goods or services, and the number of firms in the market, which can influence overall market supply levels.

Indirect tax impacts the supply side as well because it affects the cost of production; higher taxes can lead to increased costs for producers, potentially reducing supply. In contrast, income levels primarily influence consumers' purchasing power and demand for products; higher income typically leads to greater demand for goods and services, making it a factor on the demand side rather than the supply side of the market.