Assertion : Transformers are used only in alternating current source not in direct current.
Reason : Only a.c. can be stepped up or down by means of transformers.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Explanation

### Step-by-Step Solution:

1. **Understanding Transformers**:
- A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. It can either increase (step-up transformer) or decrease (step-down transformer) voltage levels.

2. **Types of Transformers**:
- There are two main types of transformers:
- **Step-Up Transformer**: Increases the voltage from primary to secondary.
- **Step-Down Transformer**: Decreases the voltage from primary to secondary.

3. **AC vs. DC**:
- Transformers operate on the principle of electromagnetic induction, which requires a changing magnetic field.
- Alternating Current (AC) provides a changing current, which creates a changing magnetic field, allowing transformers to function.
- Direct Current (DC), on the other hand, provides a constant current, resulting in a constant magnetic field. This does not induce a voltage in the secondary coil of the transformer.

4. **Assertion Analysis**:
- The assertion states that "Transformers are used only in alternating current sources and not in direct current." This is true because transformers cannot operate with DC.

5. **Reason Analysis**:
- The reason states that "Only AC can be stepped up or down by means of transformers." This is also true, as explained above, since transformers rely on the changing nature of AC to function.

6. **Conclusion**:
- Both the assertion and the reason are true. Furthermore, the reason provides a correct explanation for the assertion. Therefore, the correct option is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

### Final Answer:
Both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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