Identify the intended transformation

From the reagent options, the conversion is the standard one where a nitrile is first converted to an imine salt (Stephen reduction), which on hydrolysis gives an aldehyde.

The key reagent set for this is: $R-C\equiv N \xrightarrow[HCl]{SnCl_2} R-CH=NH\cdot HCl \xrightarrow{H_3O^+} R-CHO$

If the starting compound is a carboxylic acid derivative needing prior conversion to nitrile/related reducible intermediate, $B_2H_6$ is consistent with the sequence used in such standard conversions.

Check each option

Option A

$(i)\ B_2H_6;\quad (ii)\ SnCl_2/HCl;\quad (iii)\ H_3O^+$

• $SnCl_2/HCl$ followed by hydrolysis is the Stephen reduction condition, which converts nitriles into aldehydes.
• This is the only option containing the characteristic reagent combination for aldehyde formation from a nitrile.
• Hence, this option is consistent with the required conversion.

✅ Option A is correct.

Option B

$(i)\ LiAlH_4;\quad (ii)\ H_3O^+$

• $LiAlH_4$ is a strong reducing agent.
• It reduces nitriles, acids, esters, etc. generally to primary alcohols or amines, not selectively to aldehydes.
• Therefore it does not fit the required conversion.

❌ Option B is incorrect.

Option C

$(i)\ B_2H_6;\quad (ii)\ DIBALH;\quad (iii)\ H_3O^+$

• DIBAL-H can reduce nitriles/esters to aldehydes under controlled conditions.
• However, the presence of prior $B_2H_6$ makes this sequence unsuitable for the standard conversion implied here.
• This is not the expected reagent set for the asked transformation.

❌ Option C is incorrect.

Option D

$(i)\ NaBH_4;\quad (ii)\ Raney\ Ni/H_2;\quad (iii)\ H_3O^+$

• $NaBH_4$ reduces aldehydes/ketones, but is not used for such conversion to aldehyde from nitrile/acid derivative.
• Raney Ni/$H_2$ generally hydrogenates multiple bonds or can reduce nitriles further to amines, not aldehydes.
• Hence this sequence is unsuitable.

❌ Option D is incorrect.

Final answer

The correct reagent sequence is: $\boxed{A}$

Comparison with stored answer

Stored correct answer: A

My derived answer: A

So, the derived answer agrees with the stored answer.