Number of letters skipped in between adjacent letters in the series are multiples of 3.
Which of the following series observes this rule ?

Answer: A. AELPZ
A B  C  D E F  G  H  I  J  K L M  N  O P Q  R  S  T  U  V  W  X  Y Z
  3   6   3   9  

3, 6, 3, 9 are multiples of 3.