The measure of net molecular polarity is a quantity called dipole moment which is defined as the magnitude of the charge Q at either end of the molecular dipole times the distance 'r' between the charge :
`mu=Qxxr`
Molecular polarities give rise to some of the forces that occur between molecules.Such forces are termed as intermolecular forces.These molecular forces are of several different types including dipole-dipole forces, London dispersion forces, hydrogen bonds and ion-dipole forces (operate between ions and molecules).These intermolecular forces are electrical in origin and results from the mutual attraction of unlike charges or the mutual repulsion of like charges.
A formal positive charge on the central atom affect the size of orbitals.A formal positive charge on central atom will pull in all electrons towards the nucleus and this will leads to the contraction in size of orbitals.
Select the correct statement.

Options

A. In trisilyl amine , all N-Si bond lengths are bigger than that of normal N-Si single covalent bond
B. `PH_5` can undergo `sp^3d` hybridisation to have octahedral geometry
C. Dipole moment of `CH_3F` is greater than that of `CH_3Cl`
D. Increasing strength of hydrogen bonding is Cl-H - Cl lt N-H - NltO -H ---O ltF -H----F

Explanation

To solve the question, we need to analyze the statements provided in the context of the passage. The passage discusses dipole moments, intermolecular forces, and the effects of formal charges on atomic orbitals. Let's evaluate each statement step by step.

### Step-by-Step Solution:

1. **Evaluate Statement A:**
- Statement: "In trisilylamine, all N-Si bond lengths are bigger than that of normal N-Si single bonds."
- Explanation: Due to P-Pi-D-Pi delocalization, the N-Si bonds have partial double bond character, which means they are shorter than normal single bonds. Therefore, this statement is **incorrect**.

2. **Evaluate Statement B:**
- Statement: "pH5 can undergo sp3 hybridization to have octahedral."
- Explanation: The compound pH5 does not exist due to the large difference in size between s, p, and d orbitals. Therefore, this statement is also **incorrect**.

3. **Evaluate Statement C:**
- Statement: "Dipole moment of CH3F is greater than that of dipole moment of CH3Cl."
- Explanation: The dipole moment of CH3Cl is greater than that of CH3F due to greater charge separation between carbon and chlorine. Thus, this statement is **incorrect**.

4. **Evaluate Statement D:**
- Statement: "Increasing strength of hydrogen bonding is ClHCl < NHCl < OHCl < FHF."
- Explanation: The strength of hydrogen bonding depends on the size of the atom, electronegativity, and ease of donation of electron pairs. The order provided in the statement is correct as it reflects increasing strength of hydrogen bonding. Therefore, this statement is **correct**.

### Conclusion:
The correct statement is **Statement D**.