Letter Series Reasoning

First letter of each term is + 4 letters ahead of the previous term. Similarly second term is + 5 terms ahead of the previous term.

A B A, B C B, C B C, D C D, E D E.

After D, 4 letters are skipped and I follows after which 2 letters are skipped Trend flows alternatively:
D(EFGH) I (JK) L(MNOP) Q (RS) T(UVWX) Y(ZA) B(CDEF) G(HI) J.

The first letters of the triplets move 3, 4, 5, 6, 7 .... steps forward.
The second letters of the triplets move 5, 6, 7, 8, 9 ... steps forward.
The third letters of the triplets move 7, 8, 9, 10 .... steps forward.
Hence, the next triplet of alphabets is ZKW.

Clearly, the first, second and third letters of each term are respectively moved one, two and three steps forward to obtain the corresponding letters of the next term. So, the next term is EJO.

The first letter of each term is moved one step forward, the second letter is moved four steps backward and the third letter is moved four steps forward to obtain the corresponding letters of the next term.

ATTRIBUTION ⇒ TTRBUTIO
TTRIBUTIO ⇒ RIBUTIO
RIBUTIO ⇒ IBUTI
IBUTI ⇒ UTI

First letters of each group are in the order + 5, + 6, +7 and so on, whereas the last letters of each group are in the order + 6, + 7, + 8 and so on.

The first letters of the triplets move 3, 4, 5, 6, 7 .... steps forward.
The second letters of the triplets move 5, 6, 7, 8, 9 ... steps forward.
The third letters of the triplets move 7, 8, 9, 10 .... steps forward.
Hence, the next triplet of alphabets is ZKW.

Clearly, the first, second and third letters of each term are respectively moved one, two and three steps forward to obtain the corresponding letters of the next term. So, the next term is EJO.

The first letter of each term is moved one step forward, the second letter is moved four steps backward and the third letter is moved four steps forward to obtain the corresponding letters of the next term.

C is the 3rd letter, F sixth, I ninth so next letter will be 12th, i.e., L.
The middle numerics are the squares of 2, 3, 4 and so on. So next numeric would be 25.
The last letter follow the order : U is 3rd letter after R, X is 3rd after U. So, R would be 3rd letter after ‘O’.
∴ Missing term = L25O.

The letters in the first, second, third and fourth terms are respectively moved one, two, three and four steps forward to obtain the letter in the subsequent terms. the sequence followed by the numbers is +3, +5, +7, +9.

C is the 3rd letter, F sixth, I ninth so next letter will be 12th, i.e., L.
The middle numerics are the squares of 2, 3, 4 and so on. So next numeric would be 25.
The last letter follow the order : U is 3rd letter after R, X is 3rd after U. So, R would be 3rd letter after ‘O’.
∴ Missing term = L25O.

The first letter of each term is moved three steps forward and the second letter is moved three steps backward to obtain the corresponding letters of the next term.

Observe the first letter of each group. These letters are alternate letters starting from A. Hence, the first letter of the missing group will be G. Similarly, observe second and third letters of each group. Hence, the required group is GWH.

SEQUENTIAL, (S)EQUENTIAL, EQUENTI(A)(L), (E)QUENTI QUEN(T)(I)
In the given sequence one letter and two letters are lost alternately.

The first letters in odd numbered terms form series J, I, H and in even numbered terms form the series K, L, M. The sequence followed by the numbers is + 2, + 3, + 4, + 5, + 6. The third letter of each term is moved two steps backward to obtain the third letter of the next term.

The first letters in odd numbered terms form series J, I, H and in even numbered terms form the series K, L, M. The sequence followed by the numbers is + 2, + 3, + 4, + 5, + 6. The third letter of each term is moved two steps backward to obtain the third letter of the next term.

The first letter of each term is moved three steps forward and the last letter is moved two steps backward to obtain the corresponding letters of the next term. The numbers follow the sequence:
× 2 + 1, × 2 + 2, × 2 + 3, × 2 + 4.
So, 10 is wrong and must be replaced by:
(4 × 2 + 1) i.e., 9.