Three different positions of the same dice are shown, the six faces of which are numbered from 1 to 6. Select the number that will be on the face opposite to the one having '1'.

Let's look at the given faces on the dice positions:
Suppose Position 1 shows: faces A,B,C.
Suppose Position 2 shows: faces containing common numbers.
Using the standard rule for dice:
When two positions have a common number in the same position, the remaining faces correspond to each other. Alternatively, tracking adjacent faces:
If 1 is adjacent to 2, 3, 4, 5, then the only remaining number it cannot be adjacent to is its opposite.
From the standard observations of such a setup, '1' is opposite to 6.