A dice is numbered from 1 to 6 in different ways. If 2 is opposite to 3 and…

On a standard six-faced die, every face lies opposite exactly one other face and is adjacent to all four remaining faces. So a face is adjacent to another face whenever the two are not each other's designated opposite.

Here 2 is opposite 3, so the other four faces - 1, 4, 5 and 6 - must each be adjacent to both 2 and 3 (none of them can be 2's or 3's opposite, since that slot is already taken by each other). In particular, face 1 is neither 2's opposite nor 3's opposite, so 1 is necessarily adjacent to both 2 and 3, no matter how 1, 4, 5 and 6 eventually pair up into the remaining two opposite pairs.

• "1 opposite to 5" fixes one specific pairing of the remaining four faces, but the given clues leave more than one valid way to pair 1, 4, 5 and 6 into the last two opposite pairs, so this is only possible, not necessary.

• "4 opposite to 6" is likewise only one of the possible pairings left for 1, 4, 5 and 6 - it isn't forced by the given information.

• "4 is adjacent to 2 and 6" would need 4 and 6 to not be opposite, which again isn't guaranteed by the given clues.

Only the statement that 1 is adjacent to both 2 and 3 holds in every arrangement consistent with the given clues, so it is the one that is necessarily true.