AB, DEF, HIJK,?, STUVWX

b e d / f ? h / j ? l

Clearly in the first group, the second and third letters are respectively three and two steps ahead of the first letter.

A similar pattern would follow in the second and third groups.

I. Z, W, T, Q, ? and II. S, O, k, G, ?

I. Y,T, O and II. B, G, ?

I consists of 2nd, 7th and 12th letters from the end of the English alphabet, while

II consists of 2nd, 7th and 12th letters from the beginning of the English alphabet.

So, the missing letter in II is the 12th letter from the beginning of the English alphabet, which is L.

I. C, F, I, L, O, ? and II. Z, X, V, T, ?

I. 1st, 3rd, 5th, 7th, 9th, 11th terms i.e. A, B, C, D, E, ? II. 2nd, 4th, 6th, 8th, 10th terms i.e. B, D, F, H, ?

Clearly, I consists of consecutive letters while II consists of alternate letters. So, the missing letter in I is F, while that in II is J. So, the missing terms i.e. 10th and 11th terms are J and F respectively.

Note that the numbers representing the difference between the consecutive terms of the series again from a series - 2, 5, 10, 17, 26, 37, 50 - in which the pattern is +3, +5, +7, +9, +11, +13.