Question: What mode must be supported by UEs that support both S1 mode and N1 mode for interworking with E-UTRAN connected to EPC?
Option 1: Single-registration mode
Option 2: Dual-registration mode
Option 3: Either single-registration or dual-registration mode
Option 4: Neither single-registration nor dual-registration mode
Explanation: According to the specifications, support of single-registration mode is mandatory for UEs supporting both S1 mode and N1 mode. This ensures that the UE can operate in a compatible manner with E-UTRAN connected to EPC.