B. Email-to-Case

Explanation:
To ensure that an email quick action is displayed on the case feed, the Email-to-Case feature must be enabled.

Email-to-Case is a Salesforce feature that allows users to create cases directly from emails. When an email is received, the Email-to-Case feature can automatically create a case and populate the case fields with information from the email.

When the Email-to-Case feature is enabled, email quick actions are automatically added to the case feed. This allows users to quickly and easily send an email in response to a case without having to navigate to a separate email client.

Enabling Email Notifications, Email Templates, or Email Alerts would not ensure that the email quick action is displayed on the case feed. These features are not directly related to the display of quick actions on the case feed.

"You must have Email-to-Case enabled to use the Send Email quick action on the Cases object."

https://help.salesforce.com/s/articleView?id=sf.case_interaction_send_email_quick_action_create.htm&type=5

D its the correct answer