Explanation
The correct answer is B. Spanning-tree instance VLAN mapping.
To load-balance VLANs with MSTP, you need to configure the same VLAN-to-instance mapping on all switches in the same MST region. This means that you need to assign different VLANs to different MST instances, and then adjust the spanning tree parameters (such as priority, cost, or port role) for each instance to achieve the desired load balancing. For example, you can make one switch the root for instance 1 and another switch the root for instance 2, and then map half of the VLANs to instance 1 and the other half to instance 2.
According to the Cisco document Understand the Multiple Spanning Tree Protocol (802.1s), one of the steps to configure MST is:
Split your set of VLANs into more instances and configure different MST settings for each of these instances. In order to easily achieve this, elect Bridge D1 to be the root for VLANs 501 through 1000, and Bridge D2 to be the root for VLANs 1 through 500. These statements are true for this configuration:
Switch D1(config)#spanning-tree mst configuration
Switch D1(config-mst)#instance 1 vlan 501-1000
Switch D1(config-mst)#exit
Switch D1(config)#spanning-tree mst 1 priority 0
Switch D2(config)#spanning-tree mst configuration
Switch D2(config-mst)#instance 2 vlan 1-500
Switch D2(config-mst)#exit
Switch D2(config)#spanning-tree mst 2 priority 0
The above commands create two MST instances, 1 and 2, and map VLANs 501-1000 to instance 1 and VLANs 1-500 to instance 2. Then, they make switch D1 the root for instance 1 and switch D2 the root for instance 2.
The other options are incorrect because:
A: Spanning-tree bpdu-guard setting is a security feature that disables a port if it receives a BPDU from an unauthorized device. It does not affect load balancing with MSTP.
C: Spanning-tree CIST mapping is not a valid command. CIST stands for Common and Internal Spanning Tree, which is the spanning tree instance that runs within an MST region and interacts with other regions or non-MST switches.
D: Spanning-tree root-guard setting is another security feature that prevents a port from becoming a root port if it receives superior BPDUs from another switch. It does not affect load balancing with MSTP.

Strict queuing is the best scheduling technology to use for voice traffic on an AOS-CX switch.
Scheduling is a mechanism that determines how packets are transmitted from different queues on an egress port. Strict queuing is a scheduling method that gives the highest priority queue absolute preference over all other queues, regardless of their size or utilization. Voice traffic should be assigned to the highest priority queue and scheduled with strict queuing to ensure minimal latency and jitter. The other options are incorrect because they are either not scheduling methods or not optimal for voice traffic.
References:
https://www.arubanetworks.com/techdocs/AOS-CX/10.04/HTML/5200-6728/bk01-ch02.html
https://www.arubanetworks.com/techdocs/AOS-CX/10.04/HTML/5200-6728/bk01-ch03.html

Explanation
These are the correct statements about access from the servers to the Core when the ISL link between the switches fails, but the keepalive interface functions. Server 1 can access the core layer via both uplinks because it is connected to VSX-A, which is still active for VLAN 10. Server 2 can also access the core layer via its uplink to VSX-B, which is still active for VLAN 10 because of Active Gateway feature. Server 1 and Server 2 can communicate with each other via the core layer because they are in the same VLAN and subnet, and their traffic can be routed through the core switches. The other statements are incorrect because they either describe scenarios that are not possible or not relevant to the question. References:
https://www.arubanetworks.com/techdocs/AOS-CX/10.04/HTML/5200-6728/bk01-

Aruba CX 6300 switches support various features to control the port access for different types of devices, such as client mode, device mode, and multidomain mode. These features can help limit the number of clients that can connect to a port and prevent unauthorized devices from accessing the network.
This is because option C shows how to configure the client limit and the auth-mode for a specific port using the interface command and the aaa authentication port-access command. The client limit specifies the maximum number of clients that can connect to a port. The auth-mode specifies the authentication mode for the port. In this case, option C sets both parameters to multi-domain mode, which allows only one voice device and one data device to be authenticated on a port
https://www.arubanetworks.com/techdocs/AOS-CX/10.10/HTML/monitoring_6300-6400/Content/Chp_LEDs/fr
https://www.arubanetworks.com/products/switches/6300-series/ 3:
https://www.arubanetworks.com/techdocs/AOS-CX/10.11/HTML/security_6200-6300-6400/Content/Chp_Port_