Which of the following is not possible?

Explanation: There are generally 4 quantum numbers defined as:

1. Principal quantum number: This is represented by n which represents the shell of the atom that it belongs to which shell.
2.  
3. Azimuthal quantum number: This is represented by l and tells whether the given element belongs to s, p, d, f, g or h subshell.
4.  
5. Magnetic quantum number: Magnetic quantum number is represented by m and tells about the magnetic values of an element and its value is given by -l to +l.
6.  
7. Spin quantum number: Spin quantum number represented by s which tells about the spin value of an element.

Hence out of all the options option C is not possible as in this case l = 0 but m is showing value -1, m generally in the range of -l to +l so this is not possible.

Final answer: Thus, option C is the correct answer. n = 2,l = 0,m = - 1 is not possible.