Which of the following describes the best custom signature for detecting the use of the word "Fortinet" in chat applications?

Creating an effective custom signature for specific content in applications, especially chat, necessitates understanding the exact format and context of the traffic. The most accurate and efficient signature is derived directly from an observed packet trace, which reveals the target string within the application's specific protocol headers or payload, minimizing false positives and negatives.

This signature is too generic, only looking for 'fortinet' over TCP from the client, which would likely result in many false positives across various TCP-based applications, not just chat.

This option is incomplete and, by including 'X-MMS-IM-Format', specifically targets an outdated chat protocol (MSN Messenger), making it ineffective for modern, diverse chat applications.

This option is also incomplete and shares the same specificity issues as option C, targeting an outdated protocol and lacking the necessary completeness for a 'best' signature.