C: Logins and users: A user account in a database can be associated with a login that is stored in the master database or can be a user name that is stored in an individual database.
A login is an individual account in the master database, to which a user account in one or more databases can be linked. With a login, the credential information for the user account is stored with the login.
A user account is an individual account in any database that may be, but does not have to be, linked to a login. With a user account that is not linked to a login, the credential information is stored with the user account.
D: db_ddladmin: Members of the db_ddladmin fixed database role can run any Data Definition Language (DDL) command (such as adding and removing columns) in a database.
Reference:
https://docs.microsoft.com/en-us/azure/azure-sql/database/logins-create-manage
https://docs.microsoft.com/en-us/sql/relational-databases/security/authentication-access/database-level-roles?view=sql-server-ver15