The correct answer is: A. 12 hours.
According to the Railway Safety Act 2005, a railway employee associated with train operation must not drink alcohol for at least 12 hours before coming to work. This is to ensure that they are not impaired by alcohol and can operate trains safely.
Option B is incorrect because it is not long enough. Option C is incorrect because it is not long enough. Option D is incorrect because it is not long enough.
It is important for railway employees to be aware of the rules on alcohol consumption and to follow them carefully. This will help to ensure the safety of passengers and staff.