The four different positions of dice are given below :

Which number is on the face opposite of 6 ?

By observing the different positions of the dice,

the adjacent faces of 6 show = 2, 3, 4, 5

From these observations, it is clear that the number opposite 6 is 1 (since the number 1 is not adjacent to 6 in any position).
Thus, the correct answer is (a).