Assertion : Step-down transformer can also be used as step-up transformer.
Reason: Ratio of voltage across primary and secondary coils is the same as ratio of respective number of turns.

Options

A. If both assertion and reason are correct and reason is a correct explanation of the assertion
B. If both assertion and reason are correct but reason is not the correct explanation of assertion
C. If assertion is correct but reason is incorrect
D. If assertion is inncorrect but reason is correct

Answer

To solve the question, we need to analyze the assertion and reason provided:

**Assertion:** A step-down transformer can also be used as a step-up transformer.
**Reason:** The ratio of voltage across primary and secondary coils is the same as the ratio of respective number of turns.

### Step-by-Step Solution:

1. **Understanding Transformers:**
- A transformer consists of two coils: the primary coil (input) and the secondary coil (output).
- The number of turns in the primary coil is denoted as \( N_1 \) and in the secondary coil as \( N_2 \).

2. **Step-Down Transformer:**
- A step-down transformer has more turns in the primary coil than in the secondary coil, i.e., \( N_1 > N_2 \).
- This means it reduces the voltage from the primary to the secondary coil. The relationship between the voltages and turns is given by:
\[ \frac{V_{input}}{V_{output}} = \frac{N_1}{N_2} \]
- Since \( N_1 > N_2 \), it follows that \( V_{output} < V_{input} \).

3. **Step-Up Transformer:**
- A step-up transformer has more turns in the secondary coil than in the primary coil, i.e., \( N_2 > N_1 \).
- This increases the voltage from the primary to the secondary coil. The relationship is:
\[ \frac{V_{input}}{V_{output}} = \frac{N_2}{N_1} \]
- Here, since \( N_2 > N_1 \), it follows that \( V_{output} > V_{input} \).

4. **Using a Step-Down Transformer as a Step-Up Transformer:**
- If we reverse the connections of the transformer, we can use a step-down transformer as a step-up transformer.
- By connecting the input voltage to the secondary coil and taking the output from the primary coil, the roles of \( N_1 \) and \( N_2 \) are effectively reversed.
- Thus, we can express the relationship as:
\[ \frac{V_{input}}{V_{output}} = \frac{N_2}{N_1} \]
- Since \( N_2 < N_1 \) in the original configuration, it now acts as a step-up transformer, where \( V_{output} > V_{input} \).

5. **Conclusion:**
- The assertion that a step-down transformer can also be used as a step-up transformer is correct.
- The reason provided, that the ratio of voltage across primary and secondary coils is the same as the ratio of respective number of turns, correctly explains why this is possible.

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

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