Assertion `:` The focal length of the mirror is `f` and distance of the object from the focus is `u` , the magnification of the mirror is `f//u`.
Reason : Magnification `=("Size of the image")/("Size of object" )`

Options

A. If both the assertion and reason are true and reason explains the assertion.
B. If both the assertion and reason are true but reason does not explain the assertion.
C. If assertion is true but reason is false.
D. If assertion is false but reason is true.

Explanation

To solve the problem, we need to analyze the assertion and reason provided regarding the magnification of a mirror.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the focal length of the mirror is `f` and the distance of the object from the focus is `u`. It claims that the magnification of the mirror is given by the formula \( \frac{f}{u} \).

2. **Understanding the Magnification**:
- Magnification (m) is defined as the ratio of the size of the image (I) to the size of the object (O):
\[ m = \frac{I}{O} \]

3. **Mirror Formula**:
- The mirror formula relates the object distance (u), image distance (v), and focal length (f) of the mirror:
\[ \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \]

4. **Finding the Image Distance**:
- Rearranging the mirror formula gives:
\[ v = \frac{fu}{u - f} \]

5. **Calculating Magnification**:
- The magnification can also be expressed in terms of the image distance and object distance:
\[ m = -\frac{v}{u} \]
- Substituting the expression for \( v \):
\[ m = -\frac{\frac{fu}{u - f}}{u} = -\frac{f}{u - f} \]

6. **Analyzing the Assertion**:
- The assertion claims that \( m = \frac{f}{u} \). However, from our calculations, we found that:
\[ m = -\frac{f}{u - f} \]
- Thus, the assertion is incorrect.

7. **Understanding the Reason**:
- The reason states that magnification is defined as the ratio of the size of the image to the size of the object. This is indeed correct.

8. **Conclusion**:
- Since the assertion is incorrect but the reason is correct, the overall statement is false. The correct relationship for magnification does not equal \( \frac{f}{u} \).

### Final Answer:
- The assertion is false, while the reason is true.