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The quantum number that specifies the orientation in space for an orbital is the magnetic quantum number.
Option 1: The principal quantum number (also known as the n quantum number) specifies the energy level of an electron in an atom, but it does not specify the orientation of the orbital.
Option 3: The azimuthal quantum number (also known as the l quantum number) specifies the shape of the orbital, such as s, p, d, or f orbitals. It does not directly specify the orientation.
Option 4: The orbital quantum number (also known as the ml quantum number) specifies the orientation of the orbital within a subshell. However, it does not specify the orientation in space.
Therefore, the correct answer is option 2: the magnetic quantum number. It specifies the orientation in space by indicating the number of orbitals within a subshell and their relative orientations.