A => when case closed, tamplate "send email" will be showed & C => turn on this checkbox, when rep close case, system will send email by template automatically.

To automate email notifications when a case is closed in Salesforce, the administrator should use the following two settings:

✅ C. Case Close Template in the Support Settings

This setting allows administrators to define an email template that is automatically sent when a case is closed. It ensures a consistent message is delivered to customers without manual effort from support reps.
✅ B. Notify Contact Checkbox on Close Page Layout

When this checkbox is added to the case close page layout, and checked by default or selected by the rep, Salesforce will send the email defined in the Case Close Template automatically.

it's B&C B. Auto-response rule step set 'or status of closed:

Auto-response rules in Salesforce can be used to send automatic email responses based on certain criteria, such as the status of a case.
By setting the rule to trigger when the Case status is set to "Closed", Salesforce will automatically send an email to the customer when the case is closed.
You would need to configure an email template to send the appropriate message to the customer.
C. Notify contact checkbox on close page layout:

The Notify Contact checkbox allows the user to send an email to the contact when the case is closed. This checkbox can be placed on the Case Page Layout, and when checked, it will trigger the automatic email notification to the contact associated with the case.
This is a straightforward way to ensure that when the case is closed, the notification is sent, reducing the manual effort of reps remembering to send emails.

The answer is A, C

* Case Close Template in the Support Settings: This feature allows the administrator to define a standard email template that will be automatically sent to the contact associated with the case when itis closed. This ensures consistency in communication and reduces the manual effort required by the support reps.
* Notify Contact Checkbox on Close Page Layout: Adding this checkbox to the case close page layout gives the support reps the option to send an email notification to the contact when closing a case. If the checkbox is checked, the email notification (defined by the case close template) will be sent.