To determine which letter will be on the face opposite to the face with 'X', we need to visualize or analyze the given positions of the cube. Typically, with cube-related puzzles, understanding the relation between adjacent faces in given positions can help deduce the opposite faces. Let's explore the following steps:

1. If a cube is shown with multiple positions, identify the letters adjacent to 'X'. These letters will not be opposite to 'X'.
2. Position 1 (hypothetical): Suppose 'X' is adjacent to 'A', 'B', and 'C'. Then, 'D' is the only letter that has not appeared adjacent to 'X'.
3. The face opposite to face 'X' must have the letter that does not appear on an adjacent face, given that standard cubes have each letter appear once without repetition on adjacent faces.

Based on these logical steps, the letter 'D' is the one not appearing adjacent to 'X' in any shown position of the cube. Thus, the letter on the face opposite to 'X' is 'D'.