Which of the following terms will replace the question mark (?) in the given series? DBOI, FEQL, ?, JKUR, LNWU

Correct — B. Rule: each letter position runs its own series — 1st +2, 2nd +3, 3rd +2, 4th +3.

First letters: D(4), F(6), ?, J(10), L(12) — step +2, so the gap is H(8).
Second letters: B(2), E(5), ?, K(11), N(14) — step +3, so H(8).
Third letters: O(15), Q(17), ?, U(21), W(23) — step +2, so S(19).
Fourth letters: I(9), L(12), ?, R(18), U(21) — step +3, so O(15).

The missing term is HHSO, option (b).

Why the others are wrong

• (a)
  IHSP — IHSP takes the first letter to I(9) and the fourth to P(16), each one step too far. The first column climbs +2 from F(6) to H(8), not +3.
• (c)
  HISO — HISO puts I(9) in the second slot. That column reads B(2), E(5), so the next value is 8, which is H.
• (d)
  HISP — HISP misses two columns at once — I where H(8) belongs and P where O(15) belongs. Both come from adding 4 instead of 3.

A four-letter alphanumeric series is four independent series stacked side by side. Number the alphabet A = 1 to Z = 26, write the terms one under another, and each letter position becomes its own arithmetic progression.

Here the four progressions run +2, +3, +2, +3, and each can be verified twice — once from the terms before the gap and once from the two after it.

Checking from both sides is the safeguard. If counting forward from FEQL and counting backward from JKUR disagree, the step size is wrong.

Key facts

• In DBOI, FEQL, HHSO, JKUR, LNWU the four letter positions advance by +2, +3, +2 and +3.
• A = 1 through Z = 26 is the numbering that turns a letter series into arithmetic.
• The missing term can be checked from both neighbours, forward from FEQL and backward from JKUR.

Common traps

• Solving only the first column and taking the first option that starts with the right letter
• Adding 4 instead of 3 by counting the starting letter as step one
• Numbering the alphabet from zero, which shifts every answer by one letter

No directly related past PYQ was found.