B: Turn meeting registration on or off
Follow these steps in the Teams admin center to turn meeting registration on or off:
1. Open the Teams admin center.
2. Select Meetings from the navigation pane.
3. Under Meetings, select Meeting Policies.
4. Either select an existing policy or create a new one.
5. Within your chosen policy, navigate to the Meeting scheduling section.
6. Turn Meeting Registration setting On or Off.
7. Select Save
C: Live events policies are used to control who in your organization can hold live events and the features that are available in the events they create. You can use the default policy or create one or more custom live events policies. After you create a custom policy, assign it to a user or groups of users in your organization.
Reference:
https://learn.microsoft.com/en-us/microsoftteams/set-up-meeting-registration
https://learn.microsoft.com/en-us/microsoftteams/teams-live-events/set-up-for-teams-live-events