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.