Final answer:
A variable number of units based on market demand should be shipped from each plant to each customer.
Step-by-step explanation:
When determining how many units XYZ should ship from each plant to each customer, the most appropriate option is C) A variable number based on market demand. The quantity of units to be shipped will depend on the demand for the product from each customer. If certain customers have higher demand, more units should be shipped to them, while customers with lower demand may receive fewer units.