Choose the odd one out.

The odd one out is B. XpD.

All the other options are pairs of letters that are in alphabetical order, with the first letter of the pair being one letter before the second letter. For example, BdE is a pair of letters that are in alphabetical order, with the first letter of the pair being B and the second letter of the pair being E.

However, XpD is not a pair of letters that are in alphabetical order. The first letter of the pair is X, which is not one letter before the second letter, which is P.

Therefore, B. XpD is the odd one out.