Four positions of a dice are shown. What number must be at the bottom face when the dice is in the position as shown in figure (a)?

3

Step 1) In figure c  and d

Step 2) Compare both the figure and we find out 6 and 5 both are common

Step 3)  Therefore 3 is the number which will be opposite to 4

Step 4) Such that 3 is the number which will be at the bottom face when the dice is in the position as shown in figure (a)