The best way to ensure that a backup copy is available for restoration of mission critical data after a disaster is to periodically test backups stored in a remote location. Testing backups is essential to verify that the backup copies are valid, complete, and recoverable. Testing backups also helps to identify any issues or errors that may affect the backup process or the restoration of data. Storing backups in a remote location is important to protect the backup copies from physical damage, theft, or unauthorized access that may occur at the primary site. Using an electronic vault for incremental backups, deploying a fully automated backup maintenance system, or using both tape and disk backup systems are not sufficient to ensure that a backup copy is available for restoration of mission critical data after a disaster, as they do not address the need for testing backups or storing them in a remote location. References: Backup and Recovery of Data: The Essential Guide | Veritas, The Truth About Data Backup for Mission-Critical Environments - DATAVERSITY.

The primary benefit of using a dry-pipe fire-suppression system rather than a wet-pipe system is that a dry- pipe system has a decreased risk of leakage, as the pipes are filled with pressurized air or nitrogen instead of water until the system is activated. A wet-pipe system has a higher risk of leakage, corrosion, and freezing. A dry-pipe system is not more effective at suppressing flames, as it uses the same water-based suppressant as a wet-pipe system. A dry-pipe system does not allow more time to abort release of the suppressant, as it has a delay of only a few seconds before the water is released. A dry-pipe system does not disperse dry chemical suppressants exclusively, as it uses water as the primary suppressant. References: CISA Review Manual (Digital Version), Chapter 5, Section 5.2.3

Decreased time for incident resolution is the best indicator that an incident management process is effective.
Incident management is a process that aims to restore normal service operation as quickly as possible after an incident, which is an unplanned interruption or reduction in quality of an IT service. Decreased time for incident resolution means that the incident management process is able to identify, analyze, respond to, and resolve incidents efficiently and effectively. The other indicators do not necessarily reflect the effectiveness of the incident management process, as they may depend on other factors such as the nature, frequency, and severity of incidents. References: CISA Review Manual, 27th Edition, page 372

A test plan for the BCP is essential to ensure that the plan is effective, updated and aligned with the current business needs and objectives. A change in organizational structure with significant impact to business processes may require a revision of the BCP and a new test plan to validate its adequacy. The lack of a test plan for the BCP for two years indicates a high risk of failure in the event ofa disaster or disruption.
Therefore, this should be the auditor's greatest concern among the given options. References:
ISACA, IT Control Objectives for Sarbanes-Oxley, 4th Edition, section 5.3.21 ISACA, CISA Review Manual, 27th Edition, chapter 5, section 5.42