If the question was stated as given then the correct answer is "12 volts untill well after the best before date". If it was given this way on an exam then the person who set it would be better employed in agriculture or road maintenance at a rather junior level and never allowed near an airplane. It reminds me of one on an RCAF tradesman's test I took.
What are the advantages of an electric current?
The obvious answer, "None at all to someone awaiting execution in Florida" wasn't in the list either.