How to add CAPTCHA on comment form of a specific content type?

Hello,

This is not possible with the CAPTCHA module and I think it is a bit too specialized to implement this in the core CAPTCHA module.

However, it would take very few lines to do it yourself in a custom "glue" module. Adding a CAPTCHA element to a form is as simple as adding a text field to a form:

$form['my_captcha_element'] = array(
  '#type' => 'captcha',
);

Also see #743056: Document how to add a CAPTCHA programmatically.
In short: you would need a simple hook_form_alter() implementation with the required logic to add the CAPTCHA element to the desired form.

hope this helps