According to the [Cisco Secure Email User Guide], you can create a text resource of type Disclaimer Template and use the code view option to insert HTML code into the text box. Then, you can use this text resource in a content filter to prepend or append the HTML message to the email body[1, p. 15-16].
The other options are not valid because:
A) Creating a text resource type of Disclaimer Template and pasting the HTML code into the text box without changing to code view will not work, as the HTML code will be treated as plain text and not rendered properly[1, p. 15].
C) Creating a text resource type of Notification Template and pasting the HTML code into the text box will not work, as Notification Templates are used for sending notifications to senders or recipients, not for modifying the email body[1, p. 17].
D) Creating a text resource type of Notification Template and changing to code view to paste the HTML code into the text box will not work, as Notification Templates are used for sending notifications to senders or recipients, not for modifying the email body[1, p. 17].