SAP's Remote Function Call (RFC) protocol is based on the Common Programming Interface for Communications (CPI-C) (B) and TCP/IP (C) standards. CPI-C is a low-level API for inter-program communication, providing a basis for the RFC protocol's communication mechanisms. TCP/IP serves as the underlying transport protocol, enabling network communication between SAP systems and between SAP systems and external systems. This combination allows RFC tofacilitate reliable, efficient communication across diverse network environments. EDI (A) and OData (D) are not directly related to the foundational technologies of SAP's RFC protocol.