I is the next letter.
E dEsEr....E
g.
E
e
the letter E
L
P
The obvious one is "L" as you start with A then add 3 letters = D, then add 1 letter = E, then add 3 letters = H, then add 1 letter = I, then add 3 letters = L.
it is ABC
G comes next in the musical note sequence. After G, the sequence starts again at A.
e