The cause of induced e.m.f. is :

Options

A. magnetic flux
B. magnetic field
C. area
D. change in magnetic flux.

Explanation

To solve the question regarding the cause of induced electromotive force (e.m.f.), we will follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Induced EMF**:
Induced electromotive force (e.m.f.) is generated in a circuit when there is a change in magnetic flux through the circuit. This phenomenon is described by Faraday's law of electromagnetic induction.

2. **Identify the Relevant Formula**:
According to Faraday's law, the induced e.m.f. (ε) in a closed loop is given by the formula:
\[ \varepsilon = -\frac{d\Phi_B}{dt} \]
where \(\Phi_B\) is the magnetic flux.

3. **Analyze the Options**:
The options provided in the question are:
- Magnetic flux
- Magnetic field
- Area
- Change in magnetic flux

4. **Evaluate Each Option**:
- **Magnetic Flux**: This is a measure of the quantity of magnetism, taking into account the strength and the extent of a magnetic field. However, it does not directly cause induced e.m.f. unless it changes.
- **Magnetic Field**: This is the field around a magnet where magnetic forces are exerted. It is not the direct cause of induced e.m.f. by itself.
- **Area**: The area is involved in calculating magnetic flux, but it does not cause induced e.m.f. on its own.
- **Change in Magnetic Flux**: This is the correct option. According to Faraday's law, it is the change in magnetic flux that induces an e.m.f. in the circuit.

5. **Conclusion**:
The main cause of induced e.m.f. is the change in magnetic flux. Therefore, the correct answer is:
**Change in magnetic flux**.

### Final Answer:
The cause of induced e.m.f. is **change in magnetic flux**.