(A): Thermal stability of the hydrides of VIA group elements decreases from `H_(2)O to `H_(2)Po`
(R): Increase in M-H bond length and decreases in bond energy

Options

A. Both A & R are true, R is the correct explanation of A
B. Both A & R are true, R is not correct explanation of A
C. A is true, R is false
D. A is false, R is true

Answer

To solve the question, we need to analyze the assertion and reason provided regarding the thermal stability of the hydrides of VIA group elements (the oxygen group) and the factors affecting it.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that the thermal stability of the hydrides of VIA group elements decreases from H₂O to H₂Po.
- This means that H₂O is more thermally stable than H₂S, H₂Se, H₂Te, and H₂Po.

2. **Understanding the Reason (R)**:
- The reason states that this decrease in thermal stability is due to an increase in the M-H bond length and a decrease in bond energy.
- M-H refers to the bond between the metal (or non-metal) and hydrogen in these hydrides.

3. **Analyzing the Trend**:
- As we move down the group from oxygen (O) to polonium (Po), the size of the atoms increases.
- Larger atoms lead to longer M-H bond lengths.
- Longer bonds generally have lower bond energy, making them less stable.

4. **Connecting Assertion and Reason**:
- The assertion is true because the thermal stability of hydrides does indeed decrease from H₂O to H₂Po.
- The reason is also true because the increase in M-H bond length and decrease in bond energy are valid explanations for the decrease in thermal stability.

5. **Conclusion**:
- Both the assertion and the reason are true, and the reason correctly explains the assertion.

### Final Answer:
Both assertion (A) and reason (R) are true, and R is the correct explanation of A.