What reagent is used for the reduction of aldehydes and ketones?

The reduction of aldehydes and ketones typically involves converting these carbonyl compounds into their corresponding alcohols. Lithium tetrahydridoaluminate, known as lithium aluminium hydride (LiAlH4), is a powerful reducing agent that donates hydride ions (H-) to the carbonyl carbon, facilitating this transformation.

This reagent is particularly effective because it can reduce both aldehydes and ketones to their respective alcohols under relatively mild conditions. It works by nucleophilically attacking the electrophilic carbon in the carbonyl group, leading to the formation of an alcohol after subsequent protonation.

In contrast, the other options do not function as suitable reagents for this specific reduction process. For instance, hydrogen peroxide is more commonly associated with oxidation reactions rather than reductions. Phosphine is used in different types of chemical transformations and does not generally serve as a reducing agent for carbonyl compounds. Sodium chloride, being a simple ionic compound, does not have reducing properties and is not typically involved in reduction reactions of aldehydes or ketones.

Thus, lithium tetrahydridoaluminate is the correct choice as it effectively reduces both aldehydes and ketones, yielding alcohols.