Final answer:
The default DNS record that all zones must have is the SOA record, which contains important information about the zone.
Step-by-step explanation:
The DNS record that all zones must have by default is the SOA record.
SOA stands for Start of Authority, and it is the first record in a DNS zone file. This record contains important information about the zone, such as the primary name server for the zone, the email address of the zone administrator, and various other parameters.
For example, an SOA record may look like this:
example.com. 86400 IN SOA ns1.example.com. admin.example.com. (2022010101 7200 3600 1209600 86400)