Which one of the following shows negative magnetic susceptibility?

The correct answer is Option (4) → Diamagnetic substances

The magnetic susceptibility \( \chi \) is defined by \( \mathbf{M}=\chi\mathbf{H} \). Its sign shows whether a material is magnetised along the applied field or against it.

Option (1) Paramagnetic substances — Incorrect. They are weakly magnetised along the field, so \( \chi \) is small and positive.

Option (2) Ferromagnetic substances — Incorrect. They are strongly magnetised along the field, so \( \chi \) is large and positive.

Option (3) Aluminium — Incorrect. Aluminium is a paramagnetic material, so its \( \chi \) is small and positive.

Option (4) Diamagnetic substances — Correct. In these materials, the magnetisation is opposite to the applied field (M and H in opposite directions). So \( \chi \) is negative. A magnet repels them weakly.

NCERT (2026-27):
Magnetic Properties of Materials – “… In terms of the susceptibility χ, a material is diamagnetic if χ is negative, para- if χ is positive and small, and ferro- if χ is large and positive.”