An organic compound 'A' with molecular formula CH₄O reacts with HI to give an alkyl iodide which than reacts with potassium cyanide to produce 'B'. Compound 'B' on reduction followed by reaction with HNO₂ gives an alcohol which on oxidation gives 'C'. Compound 'C' on oxidation followed by reaction with CH₃MgBr gives 'D' which on hydrolysis gives compound 'E' which is also an alcohol. 

Which of the following is a method of preparation of HNO₂?

HCl or H₂SO₄  + NaNO₂

The correct answer option 3. \(HCl\) or \(H_2SO_4+ NaNO_2\).

The preparation of nitrous acid (\( \text{HNO}_2 \)) involves reacting sodium nitrite (\( \text{NaNO}_2 \)) with an acid, typically \( \text{HCl} \) (hydrochloric acid) or \( \text{H}_2\text{SO}_4 \) (sulfuric acid). Here’s a detailed explanation of the process:

Preparation of Nitrous Acid \((HNO_2)\)

Reaction with Hydrochloric Acid (HCl):

\(\text{NaNO}_2 + \text{HCl} \rightarrow \text{HNO}_2 + \text{NaCl} \)

Sodium nitrite (\( \text{NaNO}_2 \)) reacts withhydrochloric acid (\( \text{HCl} \)) to produce nitrous acid (\( \text{HNO}_2 \)) and sodium chloride (\( \text{NaCl} \)). This reaction occurs in aqueous solution. \( \text{HNO}_2 \) is typically unstable and exists in equilibrium with its tautomers (\( \text{HNO}_2 \) ⇌ \( \text{HON} \)).

Reaction with Sulfuric Acid \(H_2SO_4\):

\(\text{NaNO}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{HNO}_2 + \text{Na}_2\text{SO}_4 \)

Sodium nitrite (\( \text{NaNO}_2 \)) reacts with sulfuric acid (\( \text{H}_2\text{SO}_4 \)) to produce nitrous acid (\( \text{HNO}_2 \)) and sodium sulfate (\( \text{Na}_2\text{SO}_4 \)). Similar to the reaction with hydrochloric acid, this reaction also occurs in aqueous solution.

Characteristics of Nitrous Acid \((HNO_2)\):

Chemical Nature: Nitrous acid is a weak acid that is not stable in pure form and is usually handled in aqueous solutions.

Equilibrium: In aqueous solution, \( \text{HNO}_2 \) exists in equilibrium with its tautomers, primarily \( \text{HNO}_2 \) and \( \text{HON} \).

Applications: Nitrous acid and its salts are used in various chemical reactions, including diazotization reactions to form diazonium salts, which are important intermediates in organic synthesis.

Summary: The preparation of nitrous acid (\( \text{HNO}_2 \)) involves reacting sodium nitrite (\( \text{NaNO}_2 \)) with either hydrochloric acid (\( \text{HCl} \)) or sulfuric acid (\( \text{H}_2\text{SO}_4 \)). These reactions yield nitrous acid along with the corresponding salt of the acid used (sodium chloride or sodium sulfate). Therefore, the correct method of preparation of \( \text{HNO}_2 \) from the options given is: \(HCl\) or \(H_2SO_4 + NaNO_2\).

This process is fundamental in laboratories for the synthesis of nitrous acid and its derivatives.