Which CoreDNS plugin exists to catch a configuration in which queries would be forwarded back to CoreDNS itself?

Answer: D. loop

The loop plugin detects simple forwarding loops; when it finds one the CoreDNS Pod exits, which is a common cause of CoreDNS CrashLoopBackOff.

Source: Customizing DNS Service (Kubernetes) — CoreDNS › CoreDNS ConfigMap options