Which of the following letter-clusters will replace the question mark (?) in the given series to make it logically complete? TGO, VIN, XKM, ZML, ?

Correct — D. Stack the terms and read the three letter positions as three separate series.

Rule: first letter +2, second letter +2, third letter −1.

T, V, X, Z → B, because past Z the alphabet starts again at A
G, I, K, M → O
O, N, M, L → K

The fifth term is therefore BOK, which is option (d).

Why the others are wrong

• (a)
  BOM — BOM has the first two letters right but pushes the third forward. That column runs O, N, M, L at a steady −1, so it must land on K.
• (b)
  BNN — BNN advances the middle letter by 1 instead of 2. The column G, I, K, M needs O, not N.
• (c)
  XKO — XKO simply reuses the opening of the third term, XKM. The first column has already climbed to Z by the fourth term, so it cannot fall back to X.

Letter-cluster series are column problems. Write the terms one under another, number the letters A = 1 to Z = 26, and read each column as its own arithmetic series.

Columns move independently, and mixed directions are normal — two columns climb here while the third falls.

The alphabet is circular for these items: after Z you continue at A, so Z + 2 is B. That wrap is the step to watch.

Key facts

• The first column climbs T, V, X, Z, so the fifth term opens at B.
• The second column climbs G, I, K, M in steps of 2 and reaches O.
• The third column falls O, N, M, L in steps of 1 and reaches K.

Common traps

• Stopping the alphabet at Z instead of wrapping round to A.
• Assuming every column moves in the same direction.
• Reading the columns from the wrong end of the cluster.

No directly related past PYQ was found.