An audit trail may be used to detect which of the following?

An audit trail serves as a detailed record of system activities and changes, which can track user access and actions taken within an information system. The primary purpose of an audit trail is to monitor and mitigate security threats, making it an essential tool for identifying unauthorized access to a system. By logging login attempts, changes to data, and access to sensitive information, organizations can review these logs to detect any suspicious activity indicative of unauthorized users attempting to access or manipulate data.

The other options pertain to different aspects of system and data integrity but do not align directly with the primary focus of an audit trail. Loss of data might be identified through other monitoring systems or backup solutions rather than through access logs. The presence of a virus would typically require specific virus detection software rather than an audit log. Finally, successful completion of a backup is generally tracked through backup management systems, not through audit trails designed primarily to monitor user access and actions within a system. Thus, the correct answer emphasizes the role of audit trails in security monitoring, specifically related to unauthorized access.