The correct option is A (2l+1)
The principal quantum number n, belongs to the principal shell z(K,L,M,....) which is occupied by an electron and can be any positive integer (n=1,2,3,4,........). Its location is further narrowed down by the angular momentum quantum number l, corresponding to the subshell and its general shape. The value of l is dependent on the value of n(l=0,1,2,..........,n−1). The values of l correspond to specific subshells i.e. l = 0 for s; l = 1 for p; l = 2 for d; l = 3 for f.
The magnetic quantum number(ml), further divides these subshells into orbitals and gives orientation of these orbitals in space. The number of orbitals can be determined from the angular momentum quantum number l and the maximum possible values of ml can be determined from number of orbitals given by, (2l+1).