Mark the correct set of ferromagnetic substances

Options

A. iron, cobalt and nickel
B. iron, copper and lead
C. silicon, bismuth and nickel
D. aluminum, sodium and copper.

Explanation

To determine the correct set of ferromagnetic substances, we need to understand the characteristics of ferromagnetic materials. Ferromagnetic materials are characterized by their magnetic susceptibility (χm) being greater than 1 and their permeability (μ) also being greater than 1. Common examples of ferromagnetic materials include iron, cobalt, and nickel.

### Step-by-Step Solution:

1. **Identify Ferromagnetic Materials**:
- Ferromagnetic materials are those that can be magnetized and retain their magnetic properties. The most common ferromagnetic materials are iron (Fe), cobalt (Co), and nickel (Ni).

2. **Analyze Given Options**:
- We need to evaluate the pairs of materials provided in the question to identify which ones are ferromagnetic.

3. **Evaluate Each Pair**:
- **Option 1**: Iron (Fe), Cobalt (Co), and Nickel (Ni)
- All three materials are well-known ferromagnetic substances.
- **Option 2**: Copper (Cu), Cobalt (Co)
- Copper is not a ferromagnetic material; it is actually diamagnetic. Therefore, this option is incorrect.
- **Option 3**: Bismuth (Bi), Nickel (Ni), Silicon (Si)
- Nickel is ferromagnetic, but silicon is paramagnetic and bismuth is diamagnetic. Thus, this option is also incorrect.
- **Option 4**: Any other combination that includes non-ferromagnetic materials.

4. **Conclusion**:
- The only correct set of ferromagnetic substances is **Option 1: Iron, Cobalt, and Nickel**.

### Final Answer:
The correct set of ferromagnetic substances is **Iron, Cobalt, and Nickel**.