Final answer:
The web authentication method described is digest authentication, which uses an MD5 hash of the username, password, and a server-provided nonce for security.
Step-by-step explanation:
The type of web authentication that requires the browser to prompt the user for a username and password and then sends the server on each request an MD5 hash of the username, password, and a nonce value provided by the server is known as digest authentication. This is more secure than basic authentication, which sends credentials in clear text. Unlike form-based authentication and single sign-on authentication, digest authentication is implemented at the HTTP protocol level and does not require the use of HTML forms or browser cookies.