Answer: b. It's a fair method because each result is an equally likely possible outcome.
The number of students is same as the number of the sides of the dice. In this scenario, the school principal will roll two dices three times and the number that repeated most will be chosen. For every dice, all number will have equally likely possibility so the randomization is fair.
But it will have a problem if the 6 dices give 6 different number, so there is a chance to get draw result. It doesn't have any correlation with fairness though.