Final answer:
The correct answer is A) When an economy is rapidly expanding, and inflation is increasing at an unsustainable pace.
Step-by-step explanation:
Keynesian economists argue that a government should introduce contractionary fiscal policy when an economy is rapidly expanding, and inflation is increasing at an unsustainable pace.
When an economy is expanding rapidly, there is an increase in aggregate demand which leads to high levels of inflation. To combat this, Keynesian economists recommend contractionary fiscal policy, which involves reducing government spending or increasing taxes in order to decrease aggregate demand and control inflation.