A

Step 1: Understand the classification of magnetic materials. Materials can be classified into three main categories:

• Diamagnetic: Materials that are not attracted to a magnetic field and may be slightly repelled. They have all their electrons paired.
• Paramagnetic: Materials that are weakly attracted by a magnetic field, having some unpaired electrons.
• Ferromagnetic: Materials that are strongly attracted by a magnetic field, often having many unpaired electrons (like iron).

Step 2: Analyze the given options:
- Aluminium: It is a weak paramagnetic material but can exhibit some diamagnetic characteristics.
- Copper: It is diamagnetic as it has a completely filled electron configuration (all electrons are paired).
- Iron: It is ferromagnetic because it has unpaired electrons.
- Nickel: It is also ferromagnetic, primarily due to unpaired electrons.
Step 3: Since the question asks for an example of a diamagnetic substance, and we notice 'Copper' being the only paramagnetic material listed in the choices, the correct option here should be clarified as the focus is on substances generally regarded as diamagnetic rather than merely weakly aligned in a magnetic field. Therefore, the correct answer is Copper.