It is D, the fear of dying in an atomic bomb. I actually read this work a while ago, but I think this is the answer.
"Vanishing" has the connotation of happening almost magically-disappearing while people are looking. It also shows that something is disappearing or ending very quickly. Something 'finishing' or 'ending' feels like it was supposed to happen, that it's nothing out of the ordinary. Vanishing has a much more extraordinary quality.
This supports his main idea because he is trying to prove that terrible things (like the Holocaust) happen right in front of people's eyes and they don't do anything to stop it if they are being indifferent. This ties into the act of something vanishing because it happens in magic when people are looking right at the magician or the object that disappears.
Language is a system of signs represrnting ideas to convey a message. Those symbols, which are words can be arbitrary, ambiguous, abstract representations of other phenomena. Words are not intrinsically connected to what they represent. Meanings of words can shift over time. The arbitrary character allows us to invent new words.
Language is ruled guided, simply put, verbal communication is guided by unspoken but broadly understood rules.
So I would say that option A, C and D are correct.
Meanwhile, I think that option B is incorrect since a mean of communication is understood as the tool and technology employed in order to convey a message, exchange information, ideas.