Final answer:
For the main energy level n = 1, the only possible quantum magnetic number (ml) is 0, since the angular momentum quantum number (l) can only be 0 at this level, limiting ml to a single value option B is correct.
Step-by-step explanation:
The quantum magnetic numbers (ml) present in the main energy level (n = 1) depend on the value of the angular momentum quantum number (l). For the main energy level with n = 1, the only possible value of l is 0, which means that the magnetic quantum number ml can also only be 0. This is because ml ranges from -l to +l, including 0. Therefore, when l = 0, the only value for ml is 0.
This explains why there can be two electrons in the n = 1 state, each with a different spin projection ms (+1/2 or -1/2), but identical n, l, and ml values. Thus, when a student asks which quantum magnetic numbers (ml) are present in the main energy level (n = 1), the correct answer is B. 0.