A TXT record is a type of DNS record that contains arbitrary text data that can be used for various purposes such as verification, configuration, or authentication. A TXT record can contain the DKIM public key per RFC 6376, which is used to verify the digital signature of an email message generated by the DKIM private key of the sender domain.
The other options are not valid because:
A) A CNAME record is a type of DNS record that maps an alias name to a canonical name or another alias name. It does not contain any DKIM public key information.
B) An AAAA record is a type of DNS record that maps a hostname to an IPv6 address. It does not contain any DKIM public key information.
D) A PTR record is a type of DNS record that maps an IP address to a hostname, which is the reverse of an A or AAAA record. It does not contain any DKIM public key information.