I thought the same, except that the question asks which number, not letter (despite then listing letters) so presumably it is more complicated. If you take the letters and convert them into numbers based on their position in the alphabet you get 3 1 18 18 and I can see no pattern there, because 2 of them are the same it is too small a set to get any kind of pattern from.

Was it just mis-typed and it was supposed to say which letter and not which number?