Question: What triggers a UE to store a PLMN in the list of forbidden PLMNs stored in the USIM during automatic PLMN selection mode?
Option 1: Receipt of a 'Location Update Accept' message from the network
Option 2: A registration reject with the cause 'PLMN not allowed' from the network
Option 3: Expiration of the periodic location update timer in the UE
Option 4: User manual input through the UE's interface to block a specific PLMN
Explanation: This is correct because, when a UE receives a registration reject with the cause 'PLMN not allowed' from a network during automatic PLMN selection mode, it stores the rejecting PLMN in the list of forbidden PLMNs managed within the USIM. This mechanism prevents future unnecessary registration attempts to disallowed networks, optimizing the UE's network selection process.