Final answer:
The correct SQL Server command to disable a login is 'ALTER LOGIN Demo DISABLE'. This command alters the properties of the login to prevent it from being used. Option 'ALTER LOGIN Demo DISABLE' is the valid syntax for disabling a login.
Step-by-step explanation:
The correct statement to disable a login in SQL Server is a. ALTER LOGIN Demo DISABLE. This command alters the properties of a login, which in this case is to disable it. Thus, it prevents the login from being used to connect to the server.
Option b is incomplete, option c, DROP LOGIN Demo, would completely remove the login, and option d is not a valid SQL Server command syntax.