Final answer:
To correct the error in the BPMN diagram, option C, which is to add a sequence flow to an end event after the 'Discard Errors' activity, is required.
Step-by-step explanation:
In the given BPMN diagram of a subprocess, the error can be corrected by adding a sequence flow to an end event after the 'Discard Errors' activity (option C). This ensures that the process flow after the error is properly directed towards the end event. By adding the sequence flow, the error is appropriately handled, and the process can continue.