What do you think is meant by the last quote in the text ? why does nobody believe a lair even if he telling the truth?

It depends on how much of a character that person has built as a liar. If they are just a pathological liar, then nobody will ever believe them. It's like the boy that cried wolf. They all believed him for the first few times, but when it actually happened, nobody believed him because he was lying so much. Nobody believes a liar, even if they are telling the truth, because that is the kind of character that the liar built for themselves.

to show how the cables work.

anyone who was dealing with communism in soviet union in his time had an ideal picture of equality, freedom, the struggle of the working class for a better future, the success of the aristocracy's overthrow and the emancipation of subjugation.   however, anyone who would go to soviet union to see the functioning of the communist system, would be convinced that it was about sad people, about everyday bad life, about the brutal attitude of communist leaders, about ill obsession to completely exterminate every thought on aristocracy. thus, orwell wanted to show all the absurdity, the brutality of the system, the unbearable rule of suspicious morality, the ignorant people who try to play professionally.

the answers are: b. and e.