I think it's M.
B is the second letter, C is the third, D is the 4th. The difference between these letters is one. D-G is 3 and G-J is 3 so I'd assume that the next letter is the third after J which is M.
2-3-4-7-10-13
b
g.
Please note that (a) this is a sequence of square numbes, and (b) the sequence starts at 22.
Since you are using in the arrengement the all 4 letters, then there are 4! = 4*3*2*1 = 24 permutations.
It could be any number. But it couldn't just be a 'b' it could be other letters. The letters is just a grown up way of saying a missing number. Its like saying: 10+b=11.. 11-b=10.. therefore b=1
c
B (Black). Order of snooker balls.
dadas
The sequence pattern appears to be alternating between a number and its corresponding letter in the alphabet. The numbers 1, 2, 3, 4 correspond to the letters A, B, C, D. Therefore, the next letter in the sequence would be E.
The next two letters are likely C & U. There are at least 2 patterns in these letters. FIrst is the list of vowels that make up every other letter. Then the letters alternate from the ends of the alphabet. If there is a Z, a B, then a Y, then C would likely be next, and then X. But there would be whatever vowel in sequence next.
bb
bb
18 b/c the pattern is +1,+2,+3...
bb
The next letter in the sequence is S. The pattern follows the alphabet sequence with each letter moving forward two positions.
Check out the link for the sequence.
G comes next in the musical note sequence. After G, the sequence starts again at A.