You need to ensure that users use multi-factor authentication (MFA) (DP-900)

Correct answer: B. Azure Active Directory (Azure AD) authentication.
Azure AD authentication supports multi-factor authentication (MFA), which provides an additional layer of security when connecting to an Azure SQL database. Azure AD authentication uses Azure AD identities to authenticate users, and it supports MFA through various methods, such as phone calls, text messages, or authenticator apps.
Azure AD authentication is the recommended method for authenticating to Azure SQL databases, as it provides a more secure and scalable solution compared to other authentication methods.

Here's why the other options are not the best fit:
A. Service principal authentication is typically used for server-to-server authentication and does not support MFA for user connections.
C. SQL authentication uses a username and password, which does not support MFA.
D. Certificate authentication uses a certificate to authenticate, but it does not inherently support MFA. While certificates can be used in conjunction with MFA, Azure AD authentication is a more straightforward and recommended approach for enabling MFA.