Question: For which UE categories is the support for RLC Unacknowledged Mode (RLC UM) as per TS 36.322, indicated by the `rlc-UM-r15` field, specifically applicable?
Option 1: UE Category-M
Option 2: UE Categories requiring acknowledged mode only
Option 3: UE Categories 1 to 5 (LTE Advanced)
Option 4: UEs of any ue-Category-NB
Explanation: The correct answer is Option 4 because, according to the specification, the `rlc-UM-r15` field and its indication of RLC UM support are specifically applicable for UEs categorized under any `ue-Category-NB`, which typically pertains to Narrowband IoT (NB-IoT) devices. This highlights the tailored capabilities for NB-IoT UEs in leveraging unacknowledged mode for radio link control, reflecting their design for efficient, low-bandwidth communication.