JAF, JEF, JIF, JOF, ?

In a letter-series problem where every term shares the same first and last letters, the varying letter typically cycles through a fixed sequence, most commonly the vowels in alphabetical order: A, E, I, O, U. The next term must preserve that fixed first and last letter while advancing the varying letter to the next value in that sequence.

1. List the given terms: JAF, JEF, JIF, JOF.

2. Compare the first and last letters across every term: each term begins with the same letter and ends with the same letter, so that frame is held constant across the series.

3. Track only the middle letter across the terms: A, E, I, O — the vowels in alphabetical order, each term advancing to the next vowel.

4. The next vowel after O in that sequence is U, so the next term keeps the same constant first and last letters with U in the middle.

Checking against the full vowel sequence A-E-I-O-U confirms the four given terms occupy the first four positions in order, so the fifth term must take the fifth vowel, U, while keeping the same fixed first and last letters — this matches only the option built on that same frame.