The simple subject is hours, because the simple subject of a sentence is typically only one word and is the simplest form of who is doing the verb. The complete subject would be "Twelve hours" because it includes all adjectives.
The simple subject is "hours" because The simple subject is one that has a single nucleus. The subject's core is a noun or pronoun. In this case: hours. The simple subject is distinguished from the compound subject, which is one that has two or more nuclei.
A subject can be simple even if its nucleus is plural. For example: Children play in the garden. ("Children" is a plural noun, but it is still the only core of the subject, so it is a simple subject).
So that the audience isn’t confused on the authors purpose. With a smaller area it leaves not much confusion and allows the reader to know what he/her is reading.
If you accept either C or D as the correct revision then you have to accept both of them. Hence the best way to treat the sentence is to ignore both of them.
B is not correct either. You can over punctuate.
That leaves A.
Sometimes the right answer to many things is to do nothing. My dad used to say "Don't fix what isn't broke."
