Refer to the exhibit. Which command is used to complete this configuration for QoS class-based marking?

The `set dscp ef` command is used within a class configuration under a policy-map to mark packets belonging to that class with the Expedited Forwarding (EF) Differentiated Services Code Point (DSCP) value, a common action for QoS classification.

`fair-queue` is a queuing mechanism, not a marking command, and dictates how packets are queued rather than how they are classified or marked.

`random-detect` refers to congestion avoidance mechanisms like Random Early Detection (RED) or Weighted RED (WRED), not a marking command.

`priority` is a queuing command used to assign priority queuing to a class of traffic, guaranteeing bandwidth and preference, but it is not a marking command itself.