Runts are small packets that do not meet the minimum size requirement for a valid Ethernet frame (under 64 bytes). They often occur due to a network issue, such as a duplex mismatch, which can be confirmed by checking the duplex settings on both the switch port and the connected device. If there is a mismatch, configuring both ends to the same duplex setting should resolve the issue. Other options mention possible contributors to a network issue but are less specific than a duplex mismatch when it comes to runts. Replacing cables would not be the initial best course of action without first identifying a duplex mismatch as the cause. Network congestion doesn't typically cause runts directly; instead, it may lead to drops or increased latency. Lastly, while bandwidth exhaustion can cause a variety of network problems, it is less likely to be the primary cause of runt frames.