正解:A
When a user is unable to perform a reverse DNS lookup of an IP address to a hostname, the missing DNS record type is the PTR (Pointer) record. Here's what you need to know:
PTR (Pointer) Record:
The PTR record maps an IP address to a hostname (reverse DNS lookup).
It is used to resolve IP addresses back to domain names.
For example, if you have an IP address (e.g., 192.168.1.10), the PTR record would provide the corresponding hostname (e.g., server.example.com).