Option 1: Administrator-Enforced MFA
As an admin, please follow these steps to enforce MFA for a user:

1.  Log in to admin.microsoft.com.
2.  Click on Users > Active users from the left panel.

3.  Click on Multi-Factor authentication to open a new tab.

4.  Search for the user and select the checkbox next to their name. You can select multiple users if needed.
5.  Click Enable, and then click Enable multi-factor authentication in the pop-up window.

6.  Upon their next login, the user will be prompted to follow the on-screen instructions to complete the setup.