Answer:
d. greater than 300 but less than or equal to 400 units
Explanation:
Given Weekly demand d = 50 units
Standard deviation \sigma = 20
Lead time L = 6 weeks
customer level = 90%
Z value at 90% customer service level is 1.28
Saftey stock = Z\sigma\sqrt{L} = 1.28 * 20 \sqrt{6} = 62.70
Reorder point = Demand during lead time + safety stock
Demand during lead time = L*d = 6*50 = 300
Reorder point = 300 + 62.70 = 362.70