Polar molecules are the molecules:

Options

A. acquire a dipole moment only when magnetic field is absent
B. having a permanent electric dipole moment
C. having zero dipole moment
D. acquire a dipole moment only in the presence of electric field due to displacement of charges

Answer

To solve the question regarding polar molecules, we need to analyze the characteristics of polar molecules and identify the correct option from the given choices.

### Step-by-Step Solution:

1. **Understanding Polar Molecules**:
- Polar molecules are characterized by the presence of a permanent electric dipole moment. This means that there is an uneven distribution of electron density, resulting in a molecule having a positive end and a negative end.

2. **Analyzing the Options**:
- **Option 1**: "Acquire a dipole moment only when the magnetic field is absent."
- This is incorrect because polar molecules have a permanent dipole moment regardless of the magnetic field.

- **Option 2**: "Having a permanent electric dipole moment."
- This is correct. Polar molecules, such as water (H2O), have a permanent dipole moment due to their molecular structure.

- **Option 3**: "Having a zero dipole moment."
- This is incorrect. Polar molecules cannot have a zero dipole moment as that would classify them as non-polar.

- **Option 4**: "Acquire a dipole moment only in the presence of an electric field due to the displacement of the charges."
- This describes induced dipoles, which is not a characteristic of polar molecules.

3. **Conclusion**:
- The correct answer is **Option 2**: "Having a permanent electric dipole moment."