Select the correct option that will fill in the blank and complete the series.
AEI, EIO, IOU, ______

Let's examine the letters in each cluster. Notice that all characters are English vowels written in their natural sequential order: A, E, I, O, U.
Each cluster consists of three consecutive vowels in a shifting window pattern:

First term: AEI (Starts with 1
st
vowel A)

Second term: EIO (Starts with 2
nd
vowel E)

Third term: IOU (Starts with 3
rd
vowel I)

Following this sequence, the next cluster must start with the 4
th
vowel, which is O.
The three consecutive vowels starting from O cycling back are O, U, and A.

Therefore, the missing term is OUA.