Defining the Order Number field as an External ID and marking it as Unique ensures:
External ID:
Allows the field to be used for matching during data import.
Helps in upsert operations.
Unique:
Ensures no duplicate Order Numbers exist.
Maintains data integrity.
Why Not Other Options:
Option A: Direct Lookup is not a valid field type for this scenario.
Option B: Lookup fields create relationships, not suitable here.
Option D: Indirect Lookup is used with external objects.
Reference:
External IDs:
"An external ID is a custom field that has the External ID attribute, meaning that it contains unique record identifiers from a system outside of Salesforce."
- Salesforce Help: External ID Fields