The correct option is B.
A confederate is a person who takes part in an experiment and follows closely the instructions that the experimenter give him without allowing other participants to know that he is working for the owner of the experiment.
In the question given above, all the people involve are confederate with the exception of one.
And all the confederate are saying that C is the longest line when it is clearly not the longest. Of course this will make the boy who is not a confederate very agitated and uncomfortable. He will be wondering whether he is the one who have problem or the other people {the confederate}, because all of them are saying the wrong thing.
A iPod
No idea
An umbrella is very useful
A apple
A pencil
I think it's a.organization