I would go with A.
It is definitely not length because that has nothing to do with it, the subject shouldn't affect formal or informal text. Quality is the closest but I think it is incorrect because a story that is written informally can have high quality.
Headphones should be constructed so that when you play music the volume has to stay at a safe level meaning you can't control it so people don't blow their ear drums. Some people may like their music loud and others not so much so it wouldn't be fair if they couldn't control how high or low it goes.
In the Message. The Message is Justice
I think it is C but not 100% sure
