Look at the three positions of this die. Which number belongs on the face marked ? in the fourth picture?

From the views 1 touches 3, 6, 4 and 5, so 1 is opposite 2; 4 touches 5, 1 and 2, and 3 touches 1 and 6, which places 3 opposite 4 and 5 opposite 6. In picture (iv) 6 is on top and 4 is on the right; the left face must be a neighbour of both, and turning (i) to that position shows 2.