The answer D is correct because the number of reopened tickets is the best indicator for measuring the performance of IT help desk function. Reopened tickets are tickets that have been marked as resolved by the help desk agents, but the customers are not satisfied with the resolution and reopen them for further assistance. Reopened tickets reflect the quality and effectiveness of the help deskservice, as well as the customer satisfaction level. A high number of reopened tickets indicates that the help desk agents are not resolving the issues properly, or that they are not communicating well with the customers. This can lead to customer frustration, dissatisfaction, and churn. Therefore, minimizing the number of reopened tickets is a key goal for any help desk function.

The other options are not as good as option D. Percentage of problems raised from incidents (option A) is a metric that shows how many incidents are escalated to problems, which are more complex and require root cause analysis and long-term solutions. This metric reflects the complexity and severity of the issues faced by the customers, but it does not directly measure the performance of the help desk function. Mean time to categorize tickets (option B) is a metric that shows how long it takes for the help desk agents to assign a category to each ticket, such as technical, billing, or feedback. This metric reflects the efficiency and accuracy of the help desk agents, but it does not measure the quality or effectiveness of the resolution. Number of incidents reported (option C) is a metric that shows how many issues are reported by the customers to the help desk function. This metric reflects the demand and workload of the help desk function, but it does not measure how well the issues are resolved or how satisfied the customers are.