Assertion (A): Sulphur is estimates as `BaSO_(4)` and not as `MgSO_(4)`.
Reason (R ): The ionic radius of `Mg^(2+)` is less than that of `Ba^(2+)`

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, but `(R )` is not the correct explanation of `(A)`.
C. If `(A)` is correct, but `(R )` is incorrect.
D. If `(A)` is incrrect, but `(R )` is correct.

Explanation

To solve the assertion and reason question, we will analyze both the assertion and the reason step by step.

### Step 1: Analyze the Assertion
The assertion states that "Sulphur is estimated as BaSO4 and not as MgSO4." This is true because barium sulfate (BaSO4) is commonly used for the estimation of sulfur.

### Step 2: Understand the Properties of BaSO4
Barium sulfate is insoluble in water. This insolubility allows for easier precipitation and subsequent estimation of sulfur content in the sample.

### Step 3: Analyze the Reason
The reason states that "The ionic radius of Mg^(2+) is less than that of Ba^(2+)." This statement is also true. Barium ions (Ba^(2+)) have a larger ionic radius compared to magnesium ions (Mg^(2+)).

### Step 4: Connect Reason to Assertion
While both the assertion and reason are true, the reason does not provide a correct explanation for the assertion. The reason focuses on ionic radius, which does not directly relate to the solubility of the compounds in water.

### Conclusion
Both the assertion and the reason are correct, but the reason is not the correct explanation for the assertion. Therefore, the answer to the question is that both statements are true, but the reason does not explain the assertion.

### Final Answer
Both Assertion (A) and Reason (R) are correct, but Reason (R) is not the correct explanation for Assertion (A).