When DKIM (DomainKeys Identified Mail) signing is configured on Cisco ESA, the DNS record that must be updated to load the DKIM public signing key is the TXT record. The TXT record is used to store arbitrary text information in the DNS, such as the DKIM public key, which can be retrieved by the recipients to verify the DKIM signature in the message header.

SenderBase Reputation Filtering is a feature that allows Cisco ESA to reject or throttle connections from email servers based on their reputation score, which is calculated by Talos using sensor information from various sources.

According to the [Cisco Secure Email Encryption Service Add-In User Guide], you can create an encryption profile that defines the encryption settings and options for your encrypted messages[2, p. 11]. You can also create an outgoing content filter that applies the encryption profile to the messages that match certain conditions, such as having [SECURE] in the subject header[2, p. 12]. This way, you can allow users to flag the messages that require encryption by adding [SECURE] to the subject line.
The other options are not valid because:
A) Creating an encryption profile with [SECURE] in the Subject setting and enabling encryption on the mail flow policy will not work, as the Subject setting in the encryption profile is used to specify the subject line of the encrypted message envelope, not the original message[2, p. 11].
B) Creating an outgoing content filter with no conditions and with the Encrypt and Deliver Now action configured with [SECURE] in the Subject setting will not work, as this will encrypt all outgoing messages regardless of whether they have [SECURE] in the subject line or not[2, p. 12].
D) Creating a DLP policy manager message action with encryption enabled and applying it to active DLP policies for outgoing mail will not work, as this will encrypt messages based on DLP rules that detect sensitive data in the message content, not based on user flags in the subject line.

A regular expression is a sequence of characters that defines a search pattern for text. To match a string of 123ABCDEFGHJ, you need to use the following regular expression: \d{3}[A-Z]{9}. This expression means that the string must start with three digits (\d{3}), followed by nine uppercase letters ([A-Z]{9}). This expression will match any string that has the same format as 123ABCDEFGHJ. Reference = User Guide for AsyncOS 12.0 for Cisco Email Security Appliances - GD (General Deployment) - Regular Expressions [Cisco Secure Email Gateway] - Cisco

The global setting that is configured under Cisco ESA Scan Behavior is the actions for unscannable messages due to attachment type. This setting allows the administrator to specify what action to take when a message contains an attachment that cannot be scanned by the appliance, such as encrypted or password-protected files. The possible actions are:
Deliver - Deliver the message normally.
Drop - Drop the message silently without notifying the sender or recipient.
Quarantine - Quarantine the message in a specified policy quarantine.
Bounce - Bounce the message back to the sender with a specified reason.
Reference:
Scan Behavior