Answer:
C. The ml quantum number must have a positive value.
Explanation:
The single electron in hydrogen can be found in a ground state orbital of 1s having a magnetic quantum number ml = 0.
Similarly, this one electron can be excited to other higher energy levels such as 2p, 3p, 3d etc
The magnetic quantum numbers ml for higher np sub-levels of the hydrogen atom have magnetic quantum numbers; -1, 0 and 1.
The nd sub-levels have magnetic quantum numbers; -2, -1, 0, 1, 2. We can see from the foregoing that magnetic quantum number ml for hydrogen atom is not always positive.
