Two forms of the same dice are shown below. When the number of dots on the upper face of this dice is 3, what will be the number of dots on its lower face?

When the number of dots on the upper face of the dice is 3.

In the two given views of the dice, faces with  2, 4, 5 and 6 dots are adjacent to the face with 3 dots.

Therefore, the remaining face with 1 dots must be opposite to the face with 3 dots.

Thus, the correct answer is (b).