Answer:
A. 200 units per order
Step-by-step explanation:
To solve this you have to use the economic order quantity formula:

Where:
Demand = 4,000
S= supply cost = ordering cost = 20
H= holding cost = 4

Economic Order Quantity = 200
How to Remember:
Demand per year and order cost goes in the dividend.
Holding cost goes in the divisor.