A network administrator must implement a feature that supports redundancy and scaling on the switches...

The correct answer is B. Spanning tree . At the distribution layer, switch redundancy is common because multiple paths are usually built into the design to avoid a single point of failure. The problem with redundant Layer 2 paths is that they can create switching loops, broadcast storms, and unstable MAC address tables if nothing controls them. Spanning Tree Protocol solves that problem by logically blocking one or more redundant paths while still keeping those paths available as backups.

That makes spanning tree the best fit for a feature that supports both redundancy and safe scaling . As the switched environment grows, STP helps maintain a stable topology and prevents loop-related failures. If the active path fails, a blocked redundant path can be brought into service.

The other options do not address the real need. VLAN assignment is used to segment broadcast domains, but it does not manage redundant Layer 2 paths. Port speed only affects throughput on an interface. Full duplex improves communication efficiency on a link, but it also does not provide redundancy control or loop prevention.

When a Network+ question mentions redundancy on switches and asks for the best switching feature, spanning tree is the strongest answer.