Which type of cloud service utilizes a constantly changing six-digit numerical token?

The correct answer is the type of cloud service that utilizes a constantly changing six-digit numerical token is Multi-Factor Authentication, or MFA. MFA enhances security by requiring users to provide two or more verification factors to gain access to a resource, adding an additional layer beyond the traditional username and password.

One common method of MFA is the use of time-based one-time passwords (TOTPs), which produce the six-digit numerical tokens mentioned in the question. These tokens are typically generated by an application, such as an authenticator app or sent via SMS, and they refresh periodically, usually every 30 seconds. This dynamic nature of the tokens ensures that even if someone intercepts a token, it will soon become useless, greatly improving the security of the login process.

In contrast, the other options do not specifically pertain to the use of changing numerical tokens as part of their core functions. XML is a markup language used for structuring data; TLS and SSL are cryptographic protocols used for secure communication over a computer network. While TLS and SSL may be used in conjunction with MFA for secure transmissions, they do not generate or utilize the constantly changing tokens as a method of authentication themselves.