Assertion `(A)`: Dihydrogen is prepared in the laboratory by the action of conc `H_2SO_4` on granular zinc.
Reason `(R)` : Pure hydrogen can be obtained by the action of water on sodium hydride.

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 question regarding the assertion and reason about the preparation of dihydrogen, we will analyze both statements step by step.

### Step 1: Analyze the Assertion
**Assertion (A)**: Dihydrogen is prepared in the laboratory by the action of concentrated H₂SO₄ on granular zinc.

- **Explanation**: In the laboratory, dihydrogen (H₂) is typically produced by the reaction of zinc (Zn) with dilute sulfuric acid (H₂SO₄), not concentrated sulfuric acid. When zinc reacts with dilute H₂SO₄, it produces zinc sulfate (ZnSO₄) and hydrogen gas (H₂). The reaction is as follows:
\[ Zn + H_2SO_4 (dilute) \rightarrow ZnSO_4 + H_2 \uparrow \]
- **Conclusion**: The assertion is incorrect because it states that concentrated H₂SO₄ is used, which is not the case for the preparation of dihydrogen.

### Step 2: Analyze the Reason
**Reason (R)**: Pure hydrogen can be obtained by the action of water on sodium hydride.

- **Explanation**: Sodium hydride (NaH) reacts with water (H₂O) to produce sodium hydroxide (NaOH) and hydrogen gas (H₂). The reaction is as follows:
\[ NaH + H_2O \rightarrow NaOH + H_2 \uparrow \]
- This method is known for producing high-purity hydrogen gas, often exceeding 99.95% purity.
- **Conclusion**: The reason is correct as it accurately describes a method for obtaining pure hydrogen.

### Final Conclusion
- The assertion is incorrect while the reason is correct. Therefore, the correct option is **D**: Assertion is incorrect but reason is correct.