A. Best the reason why it is not better (even though it sounds right) is because it says "out of the five cousins" there for we are comparing ben to five other people and he is the best.
In the simplest sense, positive means good or the opposite of negative. If you have a positive attitude about homework, for example, you're more likely to get positive feedback on your report card.
Answer is B... It contains all the colors of a rainbow