In the box that can be formed with the given open sheet as shown in figure (X), what will be on the opposite side for B?

In an open dice fold pattern, alternate faces are always opposite to each other.
* Face A is alternate to Face C $\to$ A is opposite to C.
* Face B is alternate to Face D $\to$ B is opposite to D.
* The remaining outer faces E and F will fold up to be opposite to each other $\to$ E is opposite to F.

Hence, the face opposite to B is D.