Which is not an Authentication Scheme for HTTP?

CAPTCHA is an example of something that is not considered an authentication scheme for HTTP. CAPTCHA is a challenge-response test used to verify that a user is human. It typically requires them to complete tasks such as identifying objects in images or solving a simple puzzle. While CAPTCHA helps prevent automated bots from accessing web services, it is not an authentication scheme like Basic Authentication, Digest Authentication, OAuth, or Bearer tokens.

In contrast, Basic Authentication, Digest Authentication, OAuth, and Bearer tokens are all methods used to authenticate users or systems by verifying their identity with credentials, such as passwords, tokens, or certificates. CAPTCHA focuses on distinguishing between human and automated access rather than verifying identity or credentials, making it distinct from HTTP authentication schemes.