Explanation:

When a process fails in an operating system, it is crucial to record the error information for diagnostic and troubleshooting purposes. This helps in identifying the cause of the failure and taking appropriate measures to prevent it from happening again.

Log file:
The correct answer to the given question is option 'C' - Log file. Most operating systems write the error information to a log file when a process fails.

What is a log file?
A log file is a record of events or actions that occur within a system or application. It stores various types of information, including error messages, warnings, and other relevant details. Log files serve as a valuable resource for system administrators and developers to analyze and debug issues.

Advantages of using log files:
- Error Tracking: Log files help in tracking errors and exceptions that occur during the execution of processes. By analyzing the error information, developers can identify the root cause and fix the issue.
- System Monitoring: Log files provide insights into the overall health and performance of a system. System administrators can monitor log files to detect anomalies, performance bottlenecks, and security breaches.
- Audit Trails: Log files serve as an audit trail by recording important events and actions. This can be useful for compliance purposes, debugging, and forensic analysis.
- Troubleshooting: Log files contain valuable information that aids in troubleshooting and resolving issues. They provide a historical record of events leading up to a problem, allowing developers to recreate the scenario and identify the cause.

Conclusion:
In conclusion, when a process fails in an operating system, most operating systems write the error information to a log file. This log file serves as a vital resource for tracking errors, system monitoring, audit trails, and troubleshooting.