What is the best way to populate the Order_Iterns.Quantity item?

You add a display item named Quantity to the Order items block of your Orders form to display the
quantity on hand for each product. Quantity is a non-base table item that should reflect the count
of an ordered product from the inventories table.
What is the best way to populate the Order_Iterns.Quantity item?

A.
Post-Query trigger

B.
Pre Query trigger

C.
When-New-Form-instance trigger

D.
When New-Item-instance trigger

E.
You need to define a master-detail relation so that the item is populated automatically.

Explanation:
This should be implemented as amaster-detail form.