Which quantum number defines the orientation of the orbital in the space around the nucleus? (A) Principal quantum number (n)
(B) Angular momentum quantum number
(C) Magnetic quantum number (ml)
(D) Spin quantum number (ms)

The quantum number that defines the orientation of an orbital in space around the nucleus is the **magnetic quantum number** \((m_l)\).

### Explanation:

1. **Principal quantum number \((n)\)**:
- It defines the energy level or shell in which an electron resides and gives an idea of the size of the orbital. It does **not** determine the orientation of the orbital.

2. **Angular momentum quantum number \((l)\)**:
- It defines the shape of the orbital (e.g., \(s\), \(p\), \(d\), or \(f\) orbitals) but does **not** specify the orientation in space.

3. **Magnetic quantum number \((m_l)\)**:
- This quantum number defines the orientation of the orbital in space relative to an external magnetic field. For a given value of \(l\), the magnetic quantum number can take values from \(-l\) to \(+l\), which corresponds to different orientations of the orbitals in space.
- For example, if \(l = 1\) (a \(p\)-orbital), \(m_l\) can be \(-1\), \(0\), or \(+1\), representing three possible orientations of the \(p\)-orbitals in space.

4. **Spin quantum number \((m_s)\)**:
- This defines the spin of an electron, either \(+\frac{1}{2}\) or \(-\frac{1}{2}\), but it does **not** affect the orientation of the orbital.

### Correct answer: **(C) Magnetic quantum number \((m_l)\)**.