Which of the following is enabled by default on an EX Series switch?

Storm control is indeed enabled by default on EX Series switches to help mitigate the effects of broadcast storms which can adversely affect network performance. Storm control works by monitoring the traffic on the switch ports and allowing administrators to set thresholds for different types of traffic (like broadcast, multicast, and unknown unicast). Once the defined threshold is exceeded, storm control can automatically suppress the excess traffic to maintain network stability and performance.

This feature is vital in environments where network congestion can lead to performance degradation and helps ensure that other critical services and applications maintain their availability. The default provisioning of storm control helps in maintaining a robust and resilient network without requiring immediate configuration by the network administrator upon deployment.

In contrast, options such as MAC move limiting, IP source guard, and dynamic ARP inspection have different default configurations or require explicit enabling by the network administrator, as they deal with specific security and traffic management policies that may not be suitable for every network environment by default.