Assertion (A) : Bismuth forms only one well characterised compound in +5 oxidation state.
Reason (R) : Elements of group-15 form compounds in +5 oxidation state.

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is NOT the correct explanation of A
C. A is true but R is false
D. A is false and R is True.

Explanation

To analyze the given assertion and reason, we will break down the concepts involved step by step.

### Step 1: Understanding the Assertion
The assertion states that "Bismuth forms only one well-characterized compound in +5 oxidation state."

- **Explanation**: Bismuth (Bi) is a group 15 element. While it can theoretically exhibit a +5 oxidation state, it is not very stable. In fact, bismuth predominantly exists in the +3 oxidation state in most of its compounds. The only well-characterized compound of bismuth in the +5 state is bismuth pentoxide (Bi2O5).

### Step 2: Understanding the Reason
The reason states that "Elements of group-15 form compounds in +5 oxidation state."

- **Explanation**: Group 15 elements, which include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi), can indeed exhibit +5 oxidation states. For example, nitrogen can form compounds like N2O5, phosphorus can form PCl5, and so on. However, the stability of the +5 oxidation state varies among these elements.

### Step 3: Analyzing the Relationship Between Assertion and Reason
Now we need to evaluate whether the reason supports the assertion.

- **Analysis**: While it is true that group 15 elements can form +5 oxidation state compounds, the assertion specifically about bismuth is that it forms only one well-characterized compound in this state. The reason does not negate the assertion but rather provides context. The inert pair effect explains why bismuth prefers to exist in the +3 state rather than the +5 state, which is a crucial point in understanding the assertion.

### Step 4: Conclusion
Based on the analysis:

- The assertion is **true**: Bismuth does form only one well-characterized compound in the +5 oxidation state.
- The reason is also **true**: Group 15 elements can form compounds in the +5 oxidation state.
- However, the reason does not directly support the assertion about bismuth specifically.

Thus, the correct conclusion is that both the assertion and reason are true, but the reason does not provide a valid explanation for the assertion.

### Final Answer
- **Assertion (A)**: True
- **Reason (R)**: True
- **Conclusion**: The reason does not explain the assertion.