C: ALTER ROLE adds or removes members to or from a database role, or changes the name of a user-defined database role.
Members of the db_owner fixed database role can perform all configuration and maintenance activities on the database, and can also drop the database in SQL Server.
D: CREATE USER adds a user to the current database.
Note: Logins are created at the server level, while users are created at the database level. In other words, a login allows you to connect to the SQL Server service (also called an instance), and permissions inside the database are granted to the database users, not the logins. The logins will be assigned to server roles (for example, serveradmin) and the database users will be assigned to roles within that database (eg. db_datareader, db_bckupoperator).
References:
https://docs.microsoft.com/en-us/sql/t-sql/statements/alter-role-transact-sql
https://docs.microsoft.com/en-us/sql/t-sql/statements/create-user-transact-sql