Which reagents convert an acid chloride to an ester?

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The main idea is nucleophilic acyl substitution at the acid chloride. The carbonyl carbon in an acid chloride is highly reactive and the chloride is a good leaving group. When an alcohol approaches, the lone pair on the alcohol’s oxygen attacks the carbonyl carbon, forming a tetrahedral intermediate. Collapse of that intermediate ejects the chloride, yielding the ester. In practice, a base or similar scavenger (to take up the HCl produced) helps drive the reaction and makes the alcohol a better nucleophile by deprotonating it.

Water would simply hydrolyze the acid chloride to the corresponding carboxylic acid, not an ester. Ammonia would replace chloride with an amide rather than an ester. A base alone doesn’t provide the alkoxy group needed to form an ester. Therefore, the combination that furnishes an ester is the acid chloride reacting with an alcohol.