Roll-Up Summary Field:
Since the relationship between Order and Order Line Item is aMaster-Detail, a Roll-Up Summary field can be used on the Order object.
The MAX function in a Roll-Up Summary field calculates the latest date (the maximum value) among all the availability dates on the Order Line Items.
Why MAX Roll-Up Summary Field Works:
The latest availability date among the Order Line Items will determine when the entire Order can be shipped.
Why Not Other Options?
AandB: There is no such formula called "LATEST" or "CEILING" applicable to date fields.
C: A COUNT Roll-Up Summary is irrelevant for calculating dates.
References:Roll-Up Summary Fields:https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta
/object_reference/roll_up_summary_fields.htm