The correct answer is Isolating the affected systems. When a compromise is detected, it's crucial to contain the incident to prevent further spread or data loss. By isolating the systems, the administrator can minimize the damage while the security team works to identify the breach's extent and secure any vulnerabilities. Revoking unnecessary credentials and disabling services are actions that may be taken as part of the remediation process, but they are not immediate priorities and would not prevent further data loss if the backdoor remains active. Analyzing logs is important for a forensic investigation, but it doesn't actively prevent further data exfiltration.