The number of letters in the terms of the given series increases by one at each step.
The first letter of each term is two steps ahead of the last letter of the preceding term.
However, each term consists of consecutive letters in order.

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.

The given sequence is a combination of two series :
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.

The series may be divided into groups as shown:
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.

The given sequence is a combination of two series:
I. C, F, I, L, O, ? and II. Z, X, V, T, ?

One letter from the beginning and one from the end of a term are removed, one by one, in alternate steps.

The given sequence is a combination of two series:
I. Z, W, T, Q, ? and II. S, O, k, G, ?

The given series may be divided into 2 groups :
I. A, B, C, D, E, F, ?, ? and II. N, O, P, ?
Clearly, the given series consists of two terms of I followed by one term of II.
The missing terms in I are G and H while the missing term in II is Q.

The given sequence is a combination of two series :
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.