Which type of DNS record is used to lookup an associated host or domain name by its IP address?

The correct choice is the PTR Record, as it is specifically designed for reverse DNS lookups. This means that it allows for the resolution of an IP address back to its associated domain name or host. When a DNS resolver performs a reverse lookup, it queries the PTR record of the IP address, which then provides the name of the host that corresponds to that IP. This is particularly useful for various applications, including email systems where verifying the domain associated with an IP address can be critical for authenticating the source of the message.

In contrast, other record types serve different purposes. An A Record is used for mapping a domain name to an IPv4 address, while an MX Record is utilized for specifying mail exchange servers for a domain, essential in email routing. The SOA Record contains administrative information about the domain, including the primary name server and the domain's serial number, but does not facilitate reverse lookups. Thus, the PTR Record is the only choice that serves the specific function of mapping an IP address back to a host or domain name.