The underlying principle of transformer is

Options

A. resonance
B. mutual induction
C. self induction
D. none of the above

Explanation

To solve the question regarding the underlying principle of a transformer, we will follow these steps:

### Step-by-Step Solution:

1. **Understanding the Transformer Structure**:
- A transformer consists of two coils, known as the primary coil and the secondary coil, wound around a soft iron core.

2. **Applying Alternating Voltage**:
- When an alternating voltage is applied to the primary coil, it causes an alternating current to flow through the coil.

3. **Generation of Magnetic Field**:
- The flow of alternating current in the primary coil generates an alternating magnetic field around it.

4. **Linking Magnetic Flux**:
- The soft iron core serves to link the magnetic flux generated by the primary coil to the secondary coil.

5. **Inducing EMF in the Secondary Coil**:
- As the magnetic flux linked to the secondary coil is also alternating (due to the alternating magnetic field from the primary coil), an electromotive force (EMF) is induced in the secondary coil according to Faraday's law of electromagnetic induction.

6. **Principle of Mutual Induction**:
- The process described above, where a change in current in one coil induces an EMF in another coil, is known as mutual induction.

7. **Conclusion**:
- Therefore, the underlying principle of a transformer is mutual induction.

### Final Answer:
The underlying principle of a transformer is **Mutual Induction** (Option 2).

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