What is defined as the presence of unpaired electrons in a substance?

Paramagnetism is characterized by the presence of unpaired electrons in a substance. These unpaired electrons have magnetic moments that are not canceled out by other electrons, allowing the substance to be attracted to an external magnetic field. When exposed to a magnetic field, paramagnetic materials align in the direction of the field, which is a result of the unpaired spins of the electrons.

In contrast, diamagnetism is present in materials that do not have unpaired electrons. These substances are generally repelled by a magnetic field due to the paired electrons that generate an opposing magnetic field. Ferromagnetism arises in materials where unpaired electrons align parallel to each other, resulting in a strong magnetic effect even without an external field. Antiferromagnetism occurs when neighboring unpaired electrons align in opposite directions, effectively canceling each other's magnetic moments.

Thus, the defining feature of paramagnetism — the presence of unpaired electrons — explains why the answer is correct, as it directly relates to the magnetic behavior of substances in the presence of a magnetic field.