Exam 350-801 topic 1 question 213 discussion

C is the correct answer because if the trust boundary is the phone, it meant that phone serves a s security guard at the trust boundary enforcing QOS policies, but anything beyond the phone, simply trust the QOS markings and queues traffic based on receive DSCP or QOS values

The correct answer is C , where there will be not QOS remark to be done by the switch as the ip phone is trust boundary , thus all the marking done by the ip phone will be accepted. https://networklessons.com/quality-of-service/how-to-configure-qos-trust-boundary-on-cisco-switches ,

C is correct Cisco itself says: QoS classification initially takes place in the Access layer; however, other QoS tools are needed to ensure voice and video quality is maintained throughout the network. In addition to traffic classification, queuing and bandwidth provisioning also ensure voice and video quality. Therefore, QoS trust boundaries should be set up so that the switch will trust the QoS markings that phones place on their own packets. Layer 2 QoS uses a mechanism called class of service (CoS), which operates on the 802.1Q VLAN. By default, QoS on a Cisco access switch is disabled. Once enabled, the switch does not trust QoS settings from a phone. Two simple commands can be entered under the global menu on a switch to enable QoS and change the trust boundary. Once it is enabled, you can use a show command to verify these settings. Example 13-1 illustrates the QoS enable and trust boundary commands and the show verification command.

D is correct. There is no "layer" switch

From the reference link: "In the picture above the trust boundary is at the Cisco IP phone, this means that we won’t remark any packets or Ethernet frames anymore at the access layer switch."

D is correct