Final answer:
The sequence is alternating between letters and numbers. The letters represent the initials of the months, and the numbers represent the number of days in each month. The next letter in the sequence would be A.
Step-by-step explanation:
The sequence is alternating between letters and numbers. The letters S, O, N, D, J, F, and M represent the initials of the months: September, October, November, December, January, February, and March. The numbers 30, 31, and 28 represent the number of days in each month. Therefore, the next letter in the sequence would be A, representing the month of April.