What action will likely fix this problem?

An expanding company is deploying leased lines between its main site and two remote sites. The bandwidth of the leased lines is 128kb/s each, terminated on different serial interfaces on the main router. These links are used for combined VOIP and data traffic. The network administrator has implemented a VOIP solution to reduce costs, and has therefore reserved sufficient bandwidth in a low latency queue on each interface for the VOIP traffic. Users now complain about bad voice quality although no drops are observed in the low latency queue.
What action will likely fix this problem?

A.
mark VOIP traffic with IP precedence 6 and configure only ‘fair-queue’ on the links

B.
configure the scheduler allocate 3000 1000 command to allow the QoS code to have enough CPU cycles

C.
enable class-based traffic shaping on the VoIP traffic class

D.
enable Layer 2 fragmentation and interleaving on the links

E.
enable Frame Relay on the links and send voice and data on different Frame Relay PVCs

Explanation:
Link Fragmentation and Interleaving

Link fragmentation and interleaving (LFI) is a Layer 2 technique in which all Layer 2 frames are broken into small, equal-size fragments, and transmitted over the link in an interleaved fashion. When fragmentation and interleaving are in effect, the network device fragments all frames waiting in the queuing system where it prioritizes smaller frames. Then, the network device sends the fragments over the link. Small frames may be scheduled behind larger frames in the WFQ system. LFI fragments all frames, which reduces the queuing delay of small frames because they are sent almost immediately. Link fragmentation reduces delay and jitter by normalizing packet sizes of larger packets in order to offer more regular transmission opportunities to the voice packets.
The following LFI mechanisms are implemented in Cisco IOS:
Multilink PPP with interleaving is by far the most common and widely used form of LFI.
FRF.11 Annex C LFI is used with Voice over Frame Relay (VoFR).
FRF.12 Frame Relay LFI is used with Frame Relay data connections.