"dia sys top" is not for troubleshooting layer 3 issues rather for troubleshooting CPU and Memory issues diagnose sys top - list of processes with most CPU get system arp - show interface, IP, MAC (physical layer)
"If you suspect that there is an IP address conflict. ... you may need to look at the ARP table" - get system arp (ans. E), and two other answers, B and C - execute ping, execute traceroute.
B: execute ping: The ping command is a fundamental tool for checking the connectivity between two devices. It sends ICMP Echo Request packets to the destination and waits for ICMP Echo Reply packets. This can help you verify if there is connectivity at the IP layer.
C: execute traceroute: The traceroute command allows you to trace the route that packets take from the source to the destination. It shows the IP addresses of routers in the path and can help identify where a packet might be dropping or encountering issues.
D: diagnose sniffer packet any: The diagnose sniffer packet any command is used to capture and analyze packets on the FortiGate device. This can be helpful in inspecting the actual packets flowing through the device, allowing you to identify any anomalies or potential issues at the packet level. These commands are valuable for troubleshooting Layer 3 issues and gaining insights into the network behavior at the IP layer.