Question: What types of source cells can be associated with a Neighbouring Relation (NR) to an External E-UTRAN cell in a Telecommunication Network?
Option 1: Only EUtranGenericCell instances for Intra-E-UTRAN NR
Option 2: UtranGenericCell and GsmCell instances, but not EUtranGenericCell instances
Option 3: EUtranGenericCell, UtranGenericCell, and GsmCell instances, depending on the type of handover (Intra-E-UTRAN, Inter-RAT from UTRAN to E-UTRAN, or Inter-RAT from GERAN to E-UTRAN)
Option 4: None of the above; NRs are bidirectional and not defined by source cell types
Explanation: This is correct because a Neighbouring Relation (NR) to an External E-UTRAN cell can originate from different source cells, including EUtranGenericCell for Intra-E-UTRAN handovers, UtranGenericCell for Inter-RAT handovers from UTRAN to E-UTRAN, and GsmCell instances for Inter-RAT handovers from GERAN to E-UTRAN, accommodating various handover scenarios for seamless connectivity across different radio access technologies.