Data backups are recoverable from an offsite location is the most important thing to verify when testing an incident response plan for recovery from a ransomware attack, as it ensures that the organization can restore its data and resume its operations without paying the ransom or losing critical information. Data backups should be performed regularly, stored securely, and tested for integrity and availability. (From CISM Review Manual 15th Edition) References: CISM Review Manual 15th Edition, page 191, section 4.3.4.1.