In the default configuration, which Contract Workspace header field is an Editable Property and can...

Solution:

The correct answer from learning.sap.com is:

Effective Date

Explanation from the site:

According to the “Setting up Document Properties” lesson:

•By default, the agreement date, effective date, and expiration date can be configured as editable document properties. These editable properties can be used only once in any given contract document. SAP Learning

Thus, among your options (A–D):

•A. Effective Date is indeed an editable property that can be used just once in a contract document.

The other fields (Commodity, Region, Supplier Name) are by default read-only document properties and can be used multiple times in a contract document. SAP Learning+1