London forces are present between _______

Options

A. HCl molecules
B. `NH_(3)` molecules
C. `H_(2)O` molecules
D. `H_(2)` molecules

Explanation

To determine in which molecule London forces are present, we first need to understand what London forces are and the conditions under which they occur.

### Step-by-Step Solution:

1. **Definition of London Forces**:
- London forces, also known as dispersion forces, are weak attractive forces that occur between molecules. They arise due to the temporary dipoles that occur when electron density in a molecule becomes unevenly distributed.

2. **Characteristics of London Forces**:
- These forces are part of the van der Waals forces.
- They are present in non-polar molecules where there is no permanent dipole moment.
- The electron density must be symmetrically distributed for London forces to be significant.

3. **Analyzing the Given Options**:
- **Option A: HCl (Hydrochloric Acid)**:
- HCl is a polar molecule due to the difference in electronegativity between hydrogen and chlorine. The electron density is not symmetrically distributed, so London forces are not present.

- **Option B: NH3 (Ammonia)**:
- NH3 is also a polar molecule. Nitrogen is more electronegative than hydrogen, leading to a dipole moment. Thus, it does not exhibit London forces.

- **Option C: H2O (Water)**:
- H2O is a highly polar molecule with a bent shape, resulting in a significant dipole moment. Therefore, it does not have London forces.

- **Option D: H2 (Hydrogen)**:
- H2 is a diatomic molecule composed of two identical hydrogen atoms. Since both atoms have the same electronegativity, the electron density is symmetrically distributed, making H2 a non-polar molecule. Therefore, London forces are present.

4. **Conclusion**:
- The molecule in which London forces are present is **H2**. Thus, the correct answer is **Option D**.

### Final Answer:
**London forces are present in H2 (Option D).**