Final answer:
The 0 first digit in Fortigate TCP state signifies the 'Closed' state, where there is no active or pending TCP connection.
Step-by-step explanation:
The first digit in the TCP state code of a Fortigate security device represents different connection states. The correct answer to the question 'What is the 0 first digit in Fortigate TCP state?' is b) Closed. In this state, no active or pending TCP connection is present. When the first digit is 0, it indicates that the connection is in the Closed state, and either has not been initiated or has been fully terminated.