which of the following allows you to generate a system generated custom mail when someone sends you a mail?

The feature you’re looking for is called an email auto‑reply/automation rule using templates. In common email systems, this is done by:

• Creating a template (also called a canned response) that contains your custom message.
• Setting up an automatic rule/filter that triggers that template whenever a new mail arrives that matches certain conditions (for example: any incoming email, or emails to a specific address, or with a specific subject).

So if you see options like:

• “Out of office / automatic reply”
• “Filters + Templates / Canned responses”
• “Auto‑reply rule”

…the correct choice for “generate a system‑generated custom mail when someone sends you a mail” is the one that combines rules/filters with a saved template/canned response.