b e d f? h j? l

The given sequence is a combination of two series:

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

The correct pattern is + 6³, + 5³, + 4³, + 3³, .....

So, 521 is wrong and must be replaced by (498 + 3³) i.e. 525.

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.

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.

The given sequence is a combination of two series:

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