How to add CAPTCHA to the Cash On Devivery (COD) method on the checkout page?

[arvindporiya92 (Aryan)]

[dmitry_fedyuk (Dmitrii Fediuk)]

The task will require ~ 100 lines of code, so to get support on the forum I would recommend to split the complex task on simpler parts.
Use other CAPTHA topics to start:

• How is a CAPTCHA checked for the frontend «Contact Us» form?
• How is a CAPTCHA checked for the frontend customer registration form?
• How is a CAPTCHA checked when a customer edits its account?
• How is a CAPTCHA checked when a customer resets its password?