Look at each group of 3 letters separately, position by position. 1st letters: A, B, ?, D, E — this is simply A, B, C, D, E, so the missing 1st letter is C. 2nd letters: C, D, ?, F, G — again running alphabet, so it's E. 3rd letters: B, C, ?, E, F — running alphabet, so it's D. Putting them together we get C, E, D → CED, which is option (4).