If you choose to use, say Q&A CAPTCHA, it will not work unless you enable a setting which says:
Enable visual confirmation for registrations:
Requires new users to enter a random code matching an image to help prevent mass registrations.
(This setting appears in two places. See below.)
Since there is no image nor any "random code" in the Q&A CAPTCHA, that text should read something like:
Enable CAPTCHA module for registrations:
Requires new users to complete the CAPTCHA module you select to help prevent mass registrations.
In a related matter, there seems to be no need to have the above setting in two places, one in User registration settings and one in CAPTCHA module settings.