Correct order of decreasing thermal stability is as

Options

A. `NH_3 gt PH_3 gt AsH_3 gt SbH_3`
B. `PH_3 gt NH_3 gt AsH_3 gt SbH_3`
C. `AsH_3 gt PH_3 gt NH_3 gt SbH_3`
D. `SbH_3 gt AsH_3 gt PH_3 gt NH_3`

Explanation

To determine the correct order of decreasing thermal stability of the hydrides of the p-block elements (specifically from group 15), we can analyze the trend based on the size of the atoms and the strength of the M-H bonds.

### Step-by-Step Solution:

1. **Identify the Hydrides**: The hydrides we are considering are NS3 (Nitrogen trihydride), PS3 (Phosphorus trihydride), AsH3 (Arsenic trihydride), and SbH3 (Antimony trihydride).

2. **Understand Thermal Stability**: Thermal stability refers to the ability of a compound to resist decomposition when heated. In this context, it relates to the strength of the M-H bond in each hydride.

3. **Analyze the Trend in Bond Strength**: As we move down the group from nitrogen to antimony, the size of the central atom (M) increases. This increase in size leads to a longer and weaker M-H bond. Consequently, the thermal stability decreases.

4. **Establish the Order of Stability**:
- **NS3**: Nitrogen is at the top of the group, and its M-H bond is the strongest, making it the most thermally stable.
- **PS3**: Phosphorus is larger than nitrogen, so its M-H bond is weaker than that of NS3, resulting in lower thermal stability.
- **AsH3**: Arsenic is larger than phosphorus, further weakening the M-H bond and decreasing thermal stability.
- **SbH3**: Antimony is the largest of the four, leading to the weakest M-H bond and the lowest thermal stability.

5. **Final Order**: Based on the analysis, the correct order of decreasing thermal stability is:
- NS3 > PS3 > AsH3 > SbH3

### Conclusion:
The correct order of decreasing thermal stability of the hydrides is NS3 > PS3 > AsH3 > SbH3.