What problem can occur if a network contains an active loop?

An active loop in a network causes the issue where frames are continually duplicated and circulated within the loop, leading to excessive network congestion. This happens because frames sent out by a device can reach the same device again repeatedly through the loop, resulting in multiple copies of the same frame being processed. As a consequence, network devices such as switches face a flood of these duplicate frames, overwhelming their capacity to manage data and ultimately leading to significant slowdowns in performance.

When duplicate frames accumulate, they also consume bandwidth that would otherwise be available for legitimate network traffic, causing delays and potentially leading to the network becoming inoperable for legitimate communication. Therefore, the answer regarding network congestion due to duplicate frames accurately reflects the problems created by an active loop.