You plan to install two SQL Server 2012 environments named Environment1 and Environment2. Your company
identifies the following availability requirements for each environment:
Environment1 must have mirroring with automatic failover implemented.
Environment2 must have AlwaysOn with automatic failover implemented.
You need to identify the minimum number of SQL Server 2012 servers that must be deployed to each
environment to ensure that all data remains available if a physical server fails. How many servers should you
identify? To answer, drag the appropriate number to the correct environment in the answer area.
Select and Place:
Answer: 
Explanation:
http://msdn.microsoft.com/en-us/library/ms189852.aspx
http://msdn.microsoft.com/en-us/library/hh510230.aspx
Correct. For mirroring, we need 3 servers because it is an ‘Automatic failover”, so a witness server/instance should be in place.
In case of AlwaysON with automatic failover, only two servers (minimum) are needed.