Final answer:
An increase in (d) income taxes would cause the aggregate demand curve to shift to the left, leading to a decrease in consumer spending, output, and the price level.
Step-by-step explanation:
An increase in which of the following would cause the aggregate demand curve to shift to the left? The correct answer is (d) income taxes. An increase in income taxes leads to a fall in consumer spending because consumers have less disposable income. This, in turn, causes the aggregate demand curve to shift to the left, resulting in a lower quantity of output and a lower price level. Among the other options, consumer optimism, population growth, and an increase in net exports would typically cause a shift to the right in the aggregate demand curve. The cost of resources does not directly shift the aggregate demand curve; it influences the aggregate supply curve.