Question: What happens when a mandatory or conditionally required (with the condition being true) Information Element (IE)/IE Group is not included in a received NBAP message on the UTRAN Iub interface?
Option 1: The receiver ignores the missing IE/IE Group and processes the rest of the message.
Option 2: An abstract syntax error occurs due to the missing IE/IE Group, halting message processing.
Option 3: The receiver sends a request for retransmission of the entire message set.
Option 4: The condition for the IE/IE Group is re-evaluated based on the sender's specification version.
Explanation: If an IE/IE Group with mandatory or conditional (with the condition being true) presence is missing, it results in an abstract syntax error. The receiver expects these elements to be present under given conditions, and their absence violates the expected message structure.