Question: What action does a receiving node take when it receives a message with a Procedure ID marked as 'Ignore IE and Notify Sender' that it cannot comprehend, according to UTRAN Iub interface NBAP signalling rules?
Option 1: The receiving node shall reject the procedure using the Error Indication procedure without notifying the sender.
Option 2: The receiving node shall ignore the procedure but initiate the Error Indication procedure to notify the sender.
Option 3: The receiving node shall comprehend the message by default if it's marked for a critical procedure.
Option 4: The receiving node shall terminate all ongoing procedures due to the incomprehensible message.
Explanation: This is correct because, according to the specified rules for handling Procedure ID criticality in UTRAN Iub interface NBAP signalling, if a message is received with 'Ignore IE and Notify Sender' marking that the node cannot comprehend, it must ignore the procedure itself but still notify the sender of the issue through the Error Indication procedure.