The pattern of the given letters is:
- A (1st letter of the alphabet)
- G (7th letter)
- L (12th letter)
- P (16th letter)
- S (19th letter)
Each letter is increasing by +6, +5, +4, +3, and +2.
Hence, the next letter would be U, which is +1 from S (19th letter).
Thus, the correct answer is (A).