Which one of the following materials is ferromagnetic?

Concept: Ferromagnetism is the property of a small group of materials in which neighbouring atomic magnetic moments align spontaneously, in the same direction, within regions called magnetic domains. Once magnetised, this alignment can be retained as residual magnetism even after an external magnetic field is removed - the basis of permanent magnets. This is distinct from diamagnetism (weak repulsion, no spontaneous alignment) and from paramagnetism (weak, temporary alignment only while an external field is applied, with no locked-in parallel domains).

Application: Among common elements, only iron, cobalt, and nickel show this spontaneous domain alignment at room temperature, because their electron configuration and crystal structure favour parallel coupling of neighbouring magnetic moments. Nickel therefore behaves as a genuine ferromagnet - it can be magnetised and will retain magnetism.

Contrast with the other materials offered:

• Wood is an organic material; like most organic matter, it shows only weak diamagnetism (a feeble repulsion from a magnetic field) and no ferromagnetic behaviour.

• Gold is a diamagnetic metal - it is weakly repelled by a magnetic field rather than showing the strong, spontaneous domain alignment that defines a ferromagnetic material.

• Manganese has unpaired d-electrons, giving it weak paramagnetism at room temperature, but unlike iron, cobalt, and nickel, its atomic moments do not lock into parallel-aligned domains, so it does not show ferromagnetism.

Cross-check: The standard list of elements that are ferromagnetic at room temperature is iron, cobalt, and nickel (with gadolinium becoming ferromagnetic only near room temperature). None of wood, gold, or manganese belongs to this list, which confirms nickel as the only ferromagnetic option offered.