You need to achieve this goal by using the minimum required permissions.Which Transact-SQL statement should you use?

You are migrating a database named Orders to a new server that runs Microsoft SQL Server 2012.
You attempt to add the [Corpnet\User1] login to thedatabase. However, you receive the following error
message:
“User already exists in current database.”
You need to configure the [Corpnet\User1] login to be able to access the Orders database and retain the
original permissions.
You need to achieve this goal by using the minimum required permissions.Which Transact-SQL statement
should you use?

A.
DROP USER [User1];
CREATE USER [Corpnet\User1] FOR LOGIN [Corpnet\User1];
ALTER ROLE [db_owner] ADD MEMBER [Corpnet\User1];

B.
ALTER SERVER ROLE [sysadmin] ADD MEMBER [Corpnet\User1];

C.
ALTER USER [Corpnet\User1] WITH LOGIN [Corpnet\User1];

D.
ALTER ROLE [db_owner] ADD MEMBER [Corpnet\User1];

Explanation:
Reference: http://msdn.microsoft.com/en-us/library/ms176060.aspx