The correct answer is C. Unpaired electrons.
Paramagnetism is a property of materials that are attracted to a magnetic field. It is caused by the presence of unpaired electrons in the material. Unpaired electrons have a magnetic moment, which means they can be aligned with a magnetic field. When a paramagnetic material is placed in a magnetic field, the unpaired electrons align with the field, causing the material to be attracted to the field.
Option A is incorrect because paired electrons do not have a magnetic moment. They are aligned in opposite directions, so their magnetic fields cancel each other out.
Option B is incorrect because completely filled electronic subshells do not have any unpaired electrons. Therefore, they do not have a magnetic moment and are not paramagnetic.
Option D is incorrect because completely vacant electronic subshells do not have any electrons. Therefore, they do not have a magnetic moment and are not paramagnetic.