The magnifying power of an optical instrument is expressed in

Options

A. m
B. `m^(-1)`
C. D
D. it has no unit

Explanation

To determine the correct expression for the magnifying power of an optical instrument, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Magnifying Power**: Magnifying power refers to the ability of an optical instrument, such as a magnifying glass or a microscope, to enlarge the appearance of an object. It is a ratio that compares the size of the image produced by the instrument to the size of the object being viewed.

2. **Identify the Options**: The options given are:
- A) Meter
- B) Meter inverse
- C) Diopter
- D) No unit

3. **Analyze Each Option**:
- **Option A (Meter)**: This is a unit of length and does not apply to magnifying power, which is a dimensionless quantity.
- **Option B (Meter inverse)**: This unit is used in some contexts (like wave numbers) but is not applicable for magnifying power.
- **Option C (Diopter)**: This is a unit of measurement used for the optical power of lenses, not for magnifying power.
- **Option D (No unit)**: Magnifying power is a ratio and is dimensionless, meaning it does not have a unit.

4. **Conclusion**: Since magnifying power is a ratio of the image size to the object size, it is expressed as a number without any units. Therefore, the correct answer is that magnifying power has no unit.

### Final Answer:
The magnifying power of an optical instrument is expressed in **no unit**.

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