To measure the magnetic moment of a bar magnet, one may use

Options

A. a langent galvanometer
B. a deflection galvanometer if the earth's horizontal field is is known
C. an oscillation magnetometer if the earth horizontal field is known
D. both deflecation and oscillation magnatormeter if the earth's horizontal field is not known

Explanation

To measure the magnetic moment of a bar magnet, we can analyze the options provided and determine which instruments can be used for this purpose.

### Step-by-Step Solution:

1. **Understanding Magnetic Moment**: The magnetic moment of a bar magnet is a vector quantity that represents the strength and direction of its magnetic field. To measure it, we need appropriate instruments that can detect magnetic fields or the effects of magnets.

2. **Evaluating Option 1: Langevin Galvanometer**:
- The Langevin galvanometer is primarily designed to measure electric current. It is not suitable for measuring the magnetic moment of a bar magnet.
- **Conclusion**: Option 1 cannot be used.

3. **Evaluating Option 2: Deflection Galvanometer**:
- A deflection galvanometer can be used to measure the deflection caused by the magnetic field of the bar magnet. If the Earth's horizontal magnetic field is known, we can calculate the magnetic moment of the bar magnet based on the deflection observed.
- **Conclusion**: Option 2 can be used.

4. **Evaluating Option 3: Oscillation Magnetometer**:
- An oscillation magnetometer measures the period of oscillation of a magnet in a magnetic field. If the Earth's horizontal magnetic field is known, the magnetic moment can be calculated from the oscillation period.
- **Conclusion**: Option 3 can be used.

5. **Evaluating Option 4: Deflection and Oscillation Magnetometer without Known Horizontal Field**:
- Even if the Earth's horizontal magnetic field is not known, both deflection and oscillation magnetometers can still be used to measure the magnetic moment. The measurements can be made relative to an unknown field, and the magnetic moment can still be determined.
- **Conclusion**: Option 4 can be used.

### Final Answer:
- **Instruments that can be used to measure the magnetic moment of a bar magnet**:
- Option 2: Deflection Galvanometer (if Earth's horizontal field is known)
- Option 3: Oscillation Magnetometer (if Earth's horizontal field is known)
- Option 4: Both Deflection and Oscillation Magnetometer (if Earth's horizontal field is not known)
- **Instrument that cannot be used**:
- Option 1: Langevin Galvanometer