Assertion `(A)`: Electrolysis of hard water produces `O_2` and `D_2`.
Reason `(R)` :Ordinary hydrogen is called protium.

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 incorrect, but (R) is correct.

Explanation

To solve the assertion and reason question, we need to analyze both statements carefully.

### Step 1: Analyze the Assertion
The assertion states: "Electrolysis of hard water produces O2 and D2."

- Hard water is primarily composed of minerals like calcium and magnesium salts. When electrolysis is performed on hard water, it typically produces oxygen gas (O2) at the anode and ordinary hydrogen gas (H2) at the cathode.
- Deuterium (D2) is an isotope of hydrogen and is not produced from the electrolysis of ordinary hard water. Instead, deuterium would be produced from the electrolysis of heavy water (D2O).

**Conclusion for Assertion**: The assertion is **incorrect** because electrolysis of hard water does not produce D2.

### Step 2: Analyze the Reason
The reason states: "Ordinary hydrogen is called protium."

- Protium is indeed the most common isotope of hydrogen, represented as H or ¹H. It has one proton and no neutrons.
- This statement is accurate and correctly defines what ordinary hydrogen is.

**Conclusion for Reason**: The reason is **correct**.

### Final Conclusion
- The assertion is incorrect, and the reason is correct. Therefore, the correct option for this question is that the assertion is false, but the reason is true.

### Summary of the Solution
- Assertion (A): Incorrect (Electrolysis of hard water produces O2 and H2, not D2).
- Reason (R): Correct (Ordinary hydrogen is indeed called protium).
- Therefore, the answer is that the assertion is false, and the reason is true.

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