Explanation
B: Setting a column to Business Required means that the default behavior of a model-driven or canvas app will enforce this requirement in the app.
C: With alternate keys you can now define a column in a Dataverse table to correspond to a unique identifier (or unique combination of columns) used by the external data store. This alternate key can be used to uniquely identify a record in Dataverse in place of the primary key. You must be able to define which columns represent a unique identity for your records. Once you identify the columns that are unique to the table, you can declare them as alternate keys through the customization user interface (UI) or in the code.
Reference:
https://docs.microsoft.com/en-us/powerapps/maker/data-platform/create-edit-field-portal
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/define-alternate-keys-entity