Final answer:
A disaster recovery plan does not cover system upgrades and hardware maintenance since those are planned IT operations rather than emergency events. The plan is crucial for protecting against unforeseen disasters and threats like natural disasters, power outages, sabotage, and viral infections.
Step-by-step explanation:
A disaster recovery plan is designed to protect and restore a company's computer systems in case of emergencies. Answering the question, a disaster recovery plan can be used for all occurrences listed except for C) system upgrades and hardware maintenance. These activities are part of routine IT operations and not unplanned disaster events.
Disaster recovery plans are essential to safeguard against events such as natural disasters like floods, fires, and earthquakes, long-term power outages, computer sabotage, and major viral infections of computer systems. These all can lead to significant data loss and downtime for a company.
Situations covered by a disaster recovery plan emphasize a company's need to prepare for unforeseeable incidents that can have immediate negative impacts on their technological and operational capabilities. The plan is essentially a “break glass in case of emergency” safeguard for unforeseen circumstances, ensuring continuity and mitigation of economic risks.