Go to the site version for smartphones.
9. Which letter should replace the question mark?
  IQ test. Fast and free online iq test # 3 with answers and solutions. Question # 09. Which letter should replace the question mark?
 

Explanation:
K = C × C + 2; D = A × A + 3; T = D × D + 4; I = B × B + 5. First letters: C, A, D, B. Opposite letter = (First letter multiplied by itself) + 2 (further 3, 4, 5). In terms of the alphabetic order of letters.