What is the function of the magnet in NMR? A) It limits the nuclear spin to certain allowed spin states.
B) It allows random orientation of nuclear magnetic moments.
C) It causes the nucleus to spin.
D) It holds the sample in place.

A) It limits the nuclear spin to certain allowed spin states.

Explanation:

A nucleus with an odd atomic number or an odd mass number has a nuclear spin, which can be observed through NMR.

A proton is the simplest nucleus, and its odd atomic number of 1 implies that it has one spin. We can visualize a spinning proton as a positively charged rotating sphere. This movement of a  charged particle is like an electric current in a wired loop. It generates a magnetic field called the magnetic moment. When a small magnetic bar is placed in the field of a larger magnet, it rotates to align itself with the field of the larger magnet, a lower energy arrangement than an orientation against the field. Quantum mechanics requires that the proton's magnetic moment is aligned with the external field or against the field. The lowest energy state with the proton aligned with the field is called the spin alpha state (α spin).  The higher energy state with the proton aligned against the external magnetic field is called the beta spin state (β spin).

In the absence of an external magnetic field, the magnetic moments of the protons have random orientations. When an external magnetic field is applied, each proton in a sample it assumes β state or α state. Because the state of spin α is lower in energy, there are more α spins than β spins.