Identify the suitable reagent for the following conversion.

The correct answer is (i) AlH(iBu)₂, (ii) H₂O.

Step-by-Step Explanation:

1. Identify the reactant and the product from the reaction scheme:
The given image shows the conversion of methyl benzoate ($C_6{H}_{5}COOC{H}_3$) into benzaldehyde ($C_6{H}_{5}CHO$). This is a selective reduction of an ester to an aldehyde.

2. Analyze the reaction behavior of each given reagent:
• $LiAl{H}_4$ followed by $H^{+}/H_{2}O$: Lithium aluminium hydride is a very strong reducing agent. It reduces esters completely to primary alcohols (in this case, yielding benzyl alcohol, $C_6{H}_{5}C{H}_{2}OH$), rather than stopping at the aldehyde stage.
• $NaB{H}_4$ followed by $H^{+}/H_{2}O$: Sodium borohydride is a mild reducing agent and is generally not reactive enough to reduce esters under standard conditions.
• $H_2/Pd-BaS{O}_4$: This reagent combination (Rosenmund catalyst) is used to reduce acid chlorides to aldehydes, but it does not reduce esters to aldehydes.
• $AlH{\left(iBu\right)}_2$ (Diisobutylaluminium hydride, DIBAL-H) followed by $H_{2}O$: DIBAL-H is a selective, bulky reducing agent. When reacted with an ester at low temperatures (typically around -78 °C), it adds one equivalent of hydride to the carbonyl carbon to form a stable tetrahedral hemiacetal-like intermediate. This intermediate does not undergo further reduction. Subsequent acidic hydrolysis with water decomposes the intermediate to yield the target aldehyde (benzaldehyde).