According to the FortiOS 7.6 Study Guide and Parallel Path Processing documentation, flow-based antivirus inspection is designed to provide security with minimal impact on performance.
First, a defining characteristic of modern flow-based AV (specifically in its " hybrid " mode) is that FortiGate buffers the whole file but transmits to the client at the same time (Statement A). This behavior allows the client to start receiving data immediately to prevent session timeouts, while the FortiGate reassembles the file in memory to perform a signature check before the final packet is released.
Second, starting with recent FortiOS versions including 7.6, flow-based inspection uses a hybrid of the scanning modes (Statement B). Previously, flow mode offered " Quick " or " Full " scans; now, it combines these techniques to offer a balance between the speed of stream-based scanning and the thoroughness of archive inspection.
Third, the primary motivation for selecting this mode is that flow-based inspection optimizes performance compared to proxy-based inspection (Statement D). It processes traffic in a single pass using the IPS engine, avoiding the overhead associated with the WAD (proxy) process. Statement C is incorrect because if a virus is detected, the last packet is withheld and the connection is reset to prevent the file from being completed. Statement E is less accurate as the IPS engine loads the AV engine to perform the task rather than acting as a " standalone " entity in the context of file scanning.