A: When 4 moles of `H_(2)` reacts with 2 moles of `O_(2)`, then 4. moles of water is formed.
R: `O_(2)` will act as limiting reagent.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion
C. If Assertion is true statement but Reason is false
D. If both Assertion and Reason are false statements

Answer

To solve the problem, we need to analyze the assertion and reason provided in the question step by step.

### Step 1: Understand the Reaction
The reaction between hydrogen (H₂) and oxygen (O₂) to form water (H₂O) can be represented by the balanced chemical equation:
\[ 2H_2 + O_2 \rightarrow 2H_2O \]

### Step 2: Determine the Stoichiometry
From the balanced equation, we can see that:
- 2 moles of H₂ react with 1 mole of O₂ to produce 2 moles of H₂O.

### Step 3: Calculate the Moles of Water Produced
Given:
- 4 moles of H₂
- 2 moles of O₂

Using stoichiometry:
- From the balanced equation, 2 moles of H₂ are required for 1 mole of O₂. Therefore, 4 moles of H₂ would require:
\[ \text{Required O}_2 = \frac{4 \text{ moles H}_2}{2} = 2 \text{ moles O}_2 \]

Since we have exactly 2 moles of O₂ available, both reactants will be completely consumed.

### Step 4: Determine the Amount of Water Formed
According to the stoichiometry of the reaction:
- 2 moles of H₂ produce 2 moles of H₂O.
- Therefore, 4 moles of H₂ will produce:
\[ \text{Water produced} = 4 \text{ moles H}_2 \rightarrow 4 \text{ moles H}_2O \]

### Step 5: Identify the Limiting Reagent
To determine the limiting reagent, we compare the available moles of each reactant:
- H₂: 4 moles available
- O₂: 2 moles available

Using the stoichiometric ratio:
- For every 2 moles of H₂, 1 mole of O₂ is needed. Thus, for 4 moles of H₂, we need 2 moles of O₂, which we have.

Since both reactants are consumed completely, neither is limiting in this case. However, if we were to consider the reaction in terms of how much of each reactant is required:
- O₂ is the limiting reagent because it is the one that will run out first in a different scenario where H₂ is in excess.

### Conclusion
- The assertion states that 4 moles of water are formed, which is true.
- The reason states that O₂ will act as the limiting reagent, which is false in this specific case because both reactants are consumed completely.

Thus, the correct answer is:
**Assertion is true, but reason is false.**

### Final Answer
**Option 3: Assertion is true but reason is false.**