This is nothing more then a very short question to check if you understand the basic 2 principles of electicity!
In this case they've used the parallel setup:
so tension remains unchanged
(answers c and d are thus eliminated)
If you suppose the consuption of electricity remains unchanged (very reasonable as no change is mentioned) we can assume the answer a is the only correct one.
I can make a similar question myself placing 2 12V/40hr batteries in serie.
a:12V/40hr
b:12V/80hr
c:24V/40hr
D:24V/80hr
What do you think is the correct answer now?