Identify the suitable reagent for the following conversion.

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

Analysis of the Reaction:
1. Reactant: The starting material shown in the image is methyl benzoate (C₆H₅COOCH₃), which is an ester.
2. Product: The product shown is benzaldehyde (C₆H₅CHO), which is an aldehyde.
3. Transformation: The conversion requires the selective reduction of an ester directly to an aldehyde without reducing it further to a primary alcohol.

Role of the Reagents:
1. (i) AlH(iBu)₂ (Diisobutylaluminum hydride / DIBAL-H) followed by (ii) H₂O:
DIBAL-H is a bulky, electrophilic reducing agent. At low temperatures (usually -78 °C), it selectively reduces esters to aldehydes. The reaction proceeds through a stable tetrahedral aluminum hemiacetal intermediate. Because of the steric bulk and low temperature, this intermediate is stable and does not collapse to an aldehyde in the reaction mixture, preventing further reduction. Upon subsequent hydrolysis (addition of H₂O), the intermediate collapses to yield the aldehyde.

2. (i) NaBH₄, (ii) H⁺/H₂O:
Sodium borohydride (NaBH₄) is a mild reducing agent. It reduces aldehydes and ketones but is generally not reactive enough to reduce esters under standard conditions.

3. H₂/Pd-BaSO₄:
This reagent combination (known as Rosenmund's catalyst) is used to reduce acid chlorides (acyl chlorides) to aldehydes, not esters.

4. (i) LiAlH₄, (ii) H⁺/H₂O:
Lithium aluminum hydride (LiAlH₄) is a powerful, non-selective reducing agent. It reduces esters completely to primary alcohols (in this case, benzyl alcohol, C₆H₅CH₂OH) rather than stopping at the aldehyde stage.

Therefore, the most suitable reagent for this conversion is (i) AlH(iBu)₂, (ii) H₂O.