Answer:
C. $500 million
Explanation:
We can compute this by calculating the new credit multiplier.
Credit multiplier = 1 / Reserve ratio
Credit multiplier = 1/0.10 = 10
This is the new credit multiplier. Which means in order to meet the reserve requirement by $50 million the total money supply will decrease by 50 * 10 = $ 500 million.
(500 / 10 = $50 million which is the desired effect required)
Hope this helps.