Securimage CAPTCHA provides a way for users of forms to verify they are human by entering a series of characters read from an image or audio sample. Compared to other CAPTCHA methods, Securimage:

is completely open-source
does not rely on a third-party server
provides an HTML5 or Flash-based audio version of the challenge, for users with visual disabilities
is highly configurable

Installation:

Drupal 8/9/10

Use composer to install the Securimage library, Version 4.0.1 or later, from
https://www.phpcaptcha.org/, like so:
composer require dapphp/securimage
Install and activate the vendor_stream_wrapper module from
https://drupal.org/project/vendor_stream_wrapper
In order to use MP3 audio format, which works in more browsers than the Flash-based playback method, the LAME audio processor (https://sourceforge.net/projects/lame/) needs to be installed on your system. Most Linux distributions include this as a package.
Activate the Securimage module.

Drupal 7

Configuration:

Use the main CAPTCHA configuration page, admin/config/people/captcha, to enable Securimage for one or more forms. Use the Securimage CAPTCHA tab, admin/config/people/captcha/securimage, to configure settings specific to Securimage.

Securimage CAPTCHA

Issues