Assertion Bond angles of `NH_(3),PH_(3),AsH_(3)` and `SbH_(3)` decrease in order as mentioned
Reasoning The central atom in each possesses a lone pair .

Options

A. If both `(A)` and `(R )` are correct and `(R )` is the correct explanation of `(A)`
B. If both `(A)` and `(R )` are correct and `(R )` is the correct explanation of `(A)`
C. If `(A)` is correct but `(R )` is incorrect
D. If `(A)` is incorrect but `(R )` is correct

Explanation

To solve the assertion and reasoning question regarding the bond angles of `NH3`, `PH3`, `AsH3`, and `SbH3`, we can break it down into a step-by-step analysis.

### Step 1: Understanding the Assertion
The assertion states that the bond angles of `NH3`, `PH3`, `AsH3`, and `SbH3` decrease in the order given. We need to analyze the bond angles of these molecules.

- **Ammonia (NH3)**: The bond angle is approximately 107.3° due to the presence of a lone pair on nitrogen, which causes repulsion and slightly reduces the bond angle from the ideal tetrahedral angle of 109.5°.
- **Phosphine (PH3)**: The bond angle is about 93.5°. The larger size of phosphorus compared to nitrogen leads to less effective lone pair-bond pair repulsion.
- **Arsenic Hydride (AsH3)**: The bond angle is around 91.8°, which is even smaller than that of PH3 due to the increased size of arsenic and the corresponding decrease in bond pair repulsion.
- **Antimony Hydride (SbH3)**: The bond angle is approximately 90°, reflecting the trend of decreasing bond angles down the group.

Thus, the bond angles decrease in the order: `NH3 > PH3 > AsH3 > SbH3`.

### Step 2: Analyzing the Reasoning
The reasoning states that the central atom in each possesses a lone pair. While this statement is true, it does not fully explain the assertion regarding the bond angles.

- All these central atoms (N, P, As, Sb) do have a lone pair, but the decrease in bond angles is primarily due to the increasing size of the central atom and the corresponding decrease in electronegativity down the group. As we move down the group, the bond pair-bond pair repulsion decreases due to the larger atomic radius, which leads to smaller bond angles.

### Conclusion
- **Assertion**: True, the bond angles decrease in the order stated.
- **Reasoning**: True, but it does not adequately explain the assertion.

### Final Answer
The correct answer is **B**: Both are correct, but R (Reasoning) is not explaining A (Assertion).

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