The correct melting point order of hydrides of group is 15 elements is:

Options

A. `NH_(3)ltPH_(3)gtAsH_(3)gtSbH_(3)`
B. `NH_(3)ltPH_(3)ltAsH_(3)ltSbH_(3)`
C. `NH_(3)gtPH_(3)gtAsH_(3)gtSbH_(3)`
D. `NH_(3)gtPH_(3)ltAsH_(3)ltSbH_(3)`

Answer

To determine the correct melting point order of the hydrides of Group 15 elements (NH₃, PH₃, AsH₃, and SbH₃), we will analyze the factors that influence their melting points.

### Step-by-Step Solution:

1. **Identify the Hydrides**: The hydrides of Group 15 elements we need to consider are:
- Ammonia (NH₃)
- Phosphine (PH₃)
- Arsine (AsH₃)
- Stibine (SbH₃)

2. **Understand the Factors Affecting Melting Point**: The melting point of a compound is influenced by the type of intermolecular forces present. For these hydrides, the key forces to consider are:
- Van der Waals forces (dispersion forces)
- Hydrogen bonding (in the case of ammonia)

3. **Analyze Molecular Mass**: As we move down the group from NH₃ to SbH₃, the molecular mass increases:
- NH₃: 17 g/mol
- PH₃: 34 g/mol
- AsH₃: 77 g/mol
- SbH₃: 124 g/mol

4. **Predict Melting Points Based on Molecular Mass**: Generally, with an increase in molecular mass, the melting point tends to increase due to stronger Van der Waals forces. Therefore, we might initially expect:
- NH₃ < PH₃ < AsH₃ < SbH₃

5. **Consider Hydrogen Bonding in Ammonia**: However, ammonia (NH₃) exhibits strong hydrogen bonding due to the presence of nitrogen, which significantly raises its melting point compared to the other hydrides. Thus, despite its lower molecular mass, NH₃ will have a higher melting point than the others.

6. **Final Melting Point Order**: Taking into account the hydrogen bonding in NH₃, the correct order of melting points is:
- NH₃ > PH₃ < AsH₃ < SbH₃

### Conclusion:
The correct melting point order of the hydrides of Group 15 elements is:
**NH₃ > PH₃ < AsH₃ < SbH₃**