Final answer:
A system administrator can look at the Login History and User Login History records to determine why a user is unable to log in to Salesforce.
Step-by-step explanation:
To determine why a user is unable to log in to Salesforce, a system administrator can look at the Login History and User Login History records. Login History provides information about all login attempts made by users, including whether the attempt was successful or not. User Login History specifically tracks the login history for individual users.
By analyzing these records, the system administrator can identify possible causes for login issues, such as incorrect credentials, locked accounts, or IP restrictions. This information can help troubleshoot and resolve the login problem.
The Setup Audit Trail and Security Health Check are also useful tools for system administrators, but they may not directly provide information about user login issues in Salesforce.