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.

The purpose of checking the Certificate Revocation List (CRL) during SMTP authentication on a Cisco Secure Email Gateway is to check if the certificate is not revoked by the issuing Certificate Authority (CA). A revoked certificate means that it is no longer valid and should not be trusted. Reference = [User Guide for AsyncOS 12.0 for Cisco Email Security Appliances - GD (General Deployment) - Configuring SMTP Authentication [Cisco Secure Email Gateway] - Cisco]

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Changing the end user quarantine access setting from None authentication to Mailbox is the correct way to enforce end user authentication before accessing quarantine. This setting requires the end users to enter their email address and password in order to access their personal quarantine on the Cisco ESA.
The other options are not valid ways to enforce end user authentication before accessing quarantine, because they do not affect the end user quarantine access setting.