Final answer:
The best demonstration of Keynesian ideas was b) World War 2 and the aftermath of WW2, as government spending resulted in significant economic growth and eradicated unemployment.
Step-by-step explanation:
According to the provided information, the best demonstration of Keynesian ideas was b) World War 2 and the aftermath of WW2. During this time, government spending increased significantly, resulting in massive deficits but also annihilating unemployment and propelling economic growth, as predicted by Keynes.
The economy that emerged after the war looked nothing like the one that existed at the beginning.