This is a letter-to-number cipher question. Mapping the sequence 1, 5, 9, 15 to letters gives: 1=A, 5=E, 9=I, 15=O. The next letter following the alphabetical vowel pattern is S (19), which is the 10th letter in the odd number series (1, 3, 5, 7, 9, 11, 13, 15, 17, 19).