A, G, L, P, S, ?

Concept: In an alphabet series, treat each letter by its position (A = 1, B = 2, ... Z = 26). When the visible terms show a related numeric pattern, such as a gap between consecutive terms that shrinks by a fixed amount, the next term is found by extending that arithmetic pattern one more step, not by guessing a letter directly.

Application

1. Convert each letter to its position: A = 1, G = 7, L = 12, P = 16, S = 19.

2. Find the gap between consecutive positions: G - A = 6, L - G = 5, P - L = 4, S - P = 3.

3. The gaps decrease by 1 at every step (6, 5, 4, 3, ...), so the next gap must be 2.

4. Add the next gap to S: 19 + 2 = 21.

5. Convert position 21 back to a letter: 21 = U.

Cross-check: Re-deriving from the start with gaps 6, 5, 4, 3, 2 reproduces every given term (1, 7, 12, 16, 19) exactly before landing on 21, confirming both the rule and the missing term.

So the missing term is U.