Letter "D"
The sequence "JFMAMJJASON" represents the first letter of each month in a calendar year. Following the pattern, the next letter in the sequence should be "D" for December. So, the complete sequence for a calendar year would be:
J - January
F - February
M - March
A - April
M - May
J - June
J - July
A - August
S - September
O - October
N - November
D - December
Therefore, "D" is the next letter in the sequence.