The process of determining the source of an issue during troubleshooting is called diagnosing. Diagnosing is the third step in the troubleshooting process, after gathering information and determining if anything has changed. Diagnosing involves analyzing the symptoms and possible causes of the problem, testing hypotheses, and identifying the root cause of the problem. Researching is the process of finding relevant information or resources to help solve a problem during troubleshooting. Researching can be done before or after diagnosing, depending on the availability and reliability of the information or resources. Sourcing is not a term used in troubleshooting, but it may refer to the process of finding or obtaining materials or components for a product or service. Triaging is not a term used in troubleshooting, but it may refer to the process of prioritizing problems or tasks based on their urgency or importance. References: The Official CompTIA IT Fundamentals (ITF+) Student Guide (Exam FC0-U61), Chapter 2: IT Concepts and Terminology1