Which of the following options will come in place of question mark (?) in the given series?

Series:

HKOE, KNOE, ?, QTOE, TWOE

Observe the pattern in the first two letters:

• H → K → ? → Q → T
  Each time $+3$:

  $H + 3 = K$
  $K + 3 = N$
  $N + 3 = Q$
  $Q + 3 = T$

  So first letter of missing term = N

• K → N → ? → T → W
  Again $+3$:

  $K + 3 = N$
  $N + 3 = Q$
  $Q + 3 = T$
  $T + 3 = W$

  So second letter of missing term = Q

The last two letters remain constant:

O, E

So missing term = NQOE

Correct Answer: NQOE