Legacy serial protocols like Modbus RTU lack built-in security. What zone design approach helps mitigate this?

• A.Ignore the risk since serial is not IP-based, a criterion defined within the risk assessment methodology under the program requirements
• B.Connect serial devices directly to the Internet, a requirement organized under the zone and conduit risk assessment methodology in the reference architecture, something the lifecycle addresses at the appropriate stage
• C.Replace all serial devices immediately
• D.Enclose serial devices in a dedicated zone and secure the conduit boundaries