For a "Complaint Against Supplier" notification (e.g., type Q2, transaction QM01):

Material Document (B): Material documents (e.g., goods receipt via MIGO) are standard references, linking the complaint to a specific receipt event (SPRO > QM > Quality Notifications > Notification Creation).

Purchasing Document (C): Purchase orders (e.g., from ME21N) are commonly referenced to tie the complaint to the supplier’s delivery or order.

WBS Element (A): Work Breakdown Structure elements are PS-related, not standard for supplier complaints.

QM Order (D): QM orders track costs but aren’t reference documents for notification creation.

Thus, "Material document" and "Purchasing document" are the correct answers.References: SAP Help Portal - "Reference Documents for Supplier Complaints"; SAP S/4HANA QM User Guide - "Notification Creation".