What will come in place of the question mark (?) in the following series? U,…

Many letter series are built from a familiar, fixed set of letters read in a particular direction. The first move is to ask which known set the visible terms belong to, and whether the set is being read forwards or backwards; once the set and direction are fixed, every position in the series is determined.

Look at the visible terms U, O, I, ?, A. All four shown letters are vowels, and there are exactly five vowels in English. Writing the vowels in their usual order gives A, E, I, O, U. Reading that order from the end towards the start (backwards) gives U, O, I, E, A. Laying this beside the series U, O, I, ?, A, the terms U, O, I and the final A line up exactly, and the only term not yet shown is the one sitting in the fourth place of the reversed list.

In the reversed vowel list U, O, I, E, A, the fourth letter is E, so the missing term is E.

Cross-check: read the completed series U, O, I, E, A from right to left and it becomes A, E, I, O, U, the five vowels in their standard order, which confirms the rule is simply the vowels written in reverse.