In 3GPP rel 15, select the network function for which pooling is supported.

In the 3GPP Release 15 architecture, pooling is a fundamental concept for scalability and resiliency, formally defined for several Network Functions (NFs). The specifications explicitly describe "AMF Sets" and "SMF Sets," which are pools of these respective NFs that can be selected to serve users and sessions. Similarly, the Session Management Function (SMF) is responsible for selecting a User Plane Function (UPF) from a pool of available UPFs to anchor a PDU session. This selection allows for flexible deployment, such as placing UPFs closer to the network edge. Therefore, AMF, SMF, and UPF all explicitly support pooling.

C. UDR: While multiple Unified Data Repository (UDR) instances can be deployed for redundancy and capacity, its high availability is typically managed via database-level clustering and replication rather than the dynamic NF set/pooling mechanism used for functions like AMF or SMF.

1. 3GPP TS 23.501 V15.3.0 (2018-12), "System architecture for the 5G System (5GS)":

Section 5.21, "AMF and SMF Sets": This section explicitly defines the concept of pooling for AMF and SMF. It states, "A set of AMFs can be grouped together in an AMF Set... A set of SMFs can be grouped together in an SMF Set." This directly supports answers A and B.

Section 5.8.2.2, "UPF selection": This section details the SMF's responsibility for selecting UPFs. It states, "The SMF is responsible for selecting the UPF(s) for a PDU Session," which is the mechanism for utilizing a pool of UPF resources. This supports answer D.

2. Afzal, B., et al. "Signaling in 5G: A Survey of the 5G-Core, Access, and User-Equipment." IEEE Communications Surveys & Tutorials, vol. 24, no. 3, 2022, pp. 1638-1691. DOI: 10.1109/COMST.2022.3181222:

Section IV-B, "Access and Mobility Management Function (AMF)": "To provide reliability and load balancing, multiple AMFs can be deployed in a pool, which is known as an AMF set." (p. 1651). This confirms AMF pooling.

Section IV-C, "Session Management Function (SMF)": "Similar to the AMF, multiple SMFs can be deployed in a pool, which is known as an SMF set." (p. 1652). This confirms SMF pooling.