Which copy of the Form W-2 is sent to the employee for their federal tax filing?

Copy B is the one sent to the employee for their federal tax filing.  

The other five copies are:

Copy A - Sent to the Social Security Administration.
Copy C - Sent to employees for their records.
Copy D - For employer's record-keeping purposes.
Copy 1 - Submitted to the State, City, or Local Tax Department.
Copy 2 - Sent to the employee for filing with the employee's state tax return.